Random access procedure fallback

ABSTRACT

This disclosure provides systems, methods and apparatus for wireless communications that support fallback from a 2-step to a 4-step random access procedures. A user equipment (UE) may establish a connection with a base station using a random access procedure. The random access procedure may be a 2-step random access procedure and reduce a number of handover exchange messages. The 2-step random access procedure may include the UE transmitting, to the base station, a first random access message including a preamble and a payload. In some implementations, the base station may receive the preamble but fail to receive or decode the payload. The random access procedure may utilize the received preamble information instead of performing retransmission of the first random access message. The base station may transmit a second random access message to the UE indicating, explicitly or implicitly, a fallback to a 4-step random access procedure for connection establishment.

CROSS REFERENCE

The present Application for Patent is a Continuation of U.S. patentapplication Ser. No. 16/746,704 by ZHANG et al., entitled “RANDOM ACCESSPROCEDURE FALLBACK” filed Jan. 17, 2020, which claims the benefit ofU.S. Provisional Patent Application No. 62/800,329 by ZHANG et al.,entitled “RANDOM ACCESS PROCEDURE FALLBACK,” filed Feb. 1, 2019,assigned to the assignee hereof, and expressly incorporated herein.

TECHNICAL FIELD

This disclosure relates to wireless communications, and morespecifically to random access procedure fallback.

DESCRIPTION OF THE RELATED TECHNOLOGY

Wireless communications systems are widely deployed to provide varioustypes of communication content such as voice, video, packet data,messaging, broadcast, and so on. These systems may be capable ofsupporting communication with multiple users by sharing the availablesystem resources (such as time, frequency, and power). Examples of suchmultiple-access systems include fourth generation (4G) systems such asLong Term Evolution (LTE) systems, LTE-Advanced (LTE-A) systems, orLTE-A Pro systems, and fifth generation (5G) systems which may bereferred to as New Radio (NR) systems. These systems may employtechnologies such as code division multiple access (CDMA), time divisionmultiple access (TDMA), frequency division multiple access (FDMA),orthogonal frequency division multiple access (OFDMA), or discreteFourier transform spread orthogonal frequency division multiplexing(DFT-S-OFDM).

A wireless multiple-access communications system may include a number ofbase stations or network access nodes, each simultaneously supportingcommunication for multiple communication devices, which may be otherwiseknown as user equipment (UE). Some wireless communications systems maysupport one or more random access procedures for communication between aUE and a base station, including an initial access to a channel, aconnection re-establishment, a handover procedure, or synchronization onthe channel. The random access procedures may involve a series ofhandshake messages exchanged between the UE and the base station. Insome implementations, the exchange may be associated with anon-contention based random access procedure and the UE may transmit oneor more messages based on a reserved preamble sequence. In some otherimplementations (such as unlicensed spectrum band operations), the UEmay perform channel sensing (such as listen-before-talk (LBT) procedure)before transmitting one or more messages on available resources of thechannel, as part on the exchange.

As demand for communication access increases, a wireless communicationssystem may support methods for reducing the number of handshake messagesexchanged between a UE and a base station. The reduced random accessprocedure may minimize potential delays for channel access, particularlyfor contention-based procedures. In some implementations, however, abase station may fail to receive or decode a data payload of theexchange. Consequently, improved techniques for performing random accessprocedures are needed.

SUMMARY

The systems, methods, and devices of this disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a method of wireless communication at a UE. Themethod may include transmitting, to a base station, a first randomaccess message of a first random access procedure, the first randomaccess message including a random access preamble and a connectionrequest, monitoring a response window of a channel to receive a secondrandom access message in response to the first random access message,the response window based on a configured timer, identifying a format ofthe second random access message based on the receiving, where theformat of the second random access message indicates one of: the firstrandom access procedure or a second random access procedure, andestablishing a connection with the base station based on the firstrandom access message, the second random access message, and theindicated one of the first random access procedure or the second randomaccess procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a UE. The apparatus may include a processor, memory in electroniccommunication with the processor, and instructions stored in the memory.The instructions may be executable by the processor to cause theapparatus to transmit, to a base station, a first random access messageof a first random access procedure, the first random access messageincluding a random access preamble and a connection request, monitor aresponse window of a channel to receive a second random access messagein response to the first random access message, the response windowbased on a configured timer, identify a format of the second randomaccess message based on the receiving, where the format of the secondrandom access message indicates one of: the first random accessprocedure or a second random access procedure, and establish aconnection with the base station based on the first random accessmessage, the second random access message, and the indicated one of thefirst random access procedure or the second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a UE. The apparatus may include means for transmitting, to a basestation, a first random access message of a first random accessprocedure, the first random access message including a random accesspreamble and a connection request, monitoring a response window of achannel to receive a second random access message in response to thefirst random access message, the response window based on a configuredtimer, identifying a format of the second random access message based onthe receiving, where the format of the second random access messageindicates one of: the first random access procedure or a second randomaccess procedure, and establishing a connection with the base stationbased on the first random access message, the second random accessmessage, and the indicated one of the first random access procedure orthe second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a non-transitory computer-readablemedium storing code for wireless communication at a UE. The code mayinclude instructions executable by a processor to transmit, to a basestation, a first random access message of a first random accessprocedure, the first random access message including a random accesspreamble and a connection request, monitor a response window of achannel to receive a second random access message in response to thefirst random access message, the response window based on a configuredtimer, identify a format of the second random access message based onthe receiving, where the format of the second random access messageindicates one of: the first random access procedure or a second randomaccess procedure, and establish a connection with the base station basedon the first random access message, the second random access message,and the indicated one of the first random access procedure or the secondrandom access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include identifying the format of thesecond random access message and further may include operations,features, means, or instructions for determining the second randomaccess message includes at least a random access response and aconnection setup message for the first random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include identifying the format of thesecond random access message and further may include operations,features, means, or instructions for determining the second randomaccess message includes at least a preamble index and an uplink grantindicating a switch from the first random access procedure to the secondrandom access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for transmitting a third random access message based on theswitch from the first random access procedure to the second randomaccess procedure, the third random access message including aretransmission of the connection request, receiving a fourth randomaccess message from the base station in response to the third randomaccess message, the fourth random access message including a connectionsetup message in response to the connection request, and whereestablishing the connection can be further based on the third randomaccess message and the fourth random access message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for initiating the response window of the channel followingthe connection request.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for initiating the response window of the channel followingthe random access preamble and prior to the connection request.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for sensing the channel prior to at least one of the randomaccess preamble or the connection request, and where the transmittingcan be based on sensing the channel, the transmitting spanning one ormore physical uplink shared channel transmit occasions.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first random accessprocedure can be a two-step random access procedure and the secondrandom access procedure can be a four-step random access procedure.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a method at a UE. The method may includetransmitting, to a base station, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a connection request, monitoring one ormore response windows of a channel to receive a second random accessmessage in response to the first random access message, selecting one ofthe first random access procedure or a second random access procedure,where the selection is based on a response window of the one or moreresponse windows over which the second random access message isreceived, and establishing a connection with the base station based onthe first random access message, the second random access message, andthe selected one of the first random access procedure or the secondrandom access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a UE. The apparatus may include a processor, memory in electroniccommunication with the processor, and instructions stored in the memory.The instructions may be executable by the processor to cause theapparatus to transmit, to a base station, a first random access messageof a first random access procedure, the first random access messageincluding a random access preamble and a connection request, monitor oneor more response windows of a channel to receive a second random accessmessage in response to the first random access message, select one ofthe first random access procedure or a second random access procedure,where the selection is based on a response window of the one or moreresponse windows over which the second random access message isreceived, and establish a connection with the base station based on thefirst random access message, the second random access message, and theselected one of the first random access procedure or the second randomaccess procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a UE. The apparatus may include means for transmitting, to a basestation, a first random access message of a first random accessprocedure, the first random access message including a random accesspreamble and a connection request, monitoring one or more responsewindows of a channel to receive a second random access message inresponse to the first random access message, selecting one of the firstrandom access procedure or a second random access procedure, where theselection is based on a response window of the one or more responsewindows over which the second random access message is received, andestablishing a connection with the base station based on the firstrandom access message, the second random access message, and theselected one of the first random access procedure or the second randomaccess procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a non-transitory computer-readablemedium storing code for wireless communication at a UE. The code mayinclude instructions executable by a processor to transmit, to a basestation, a first random access message of a first random accessprocedure, the first random access message including a random accesspreamble and a connection request, monitor one or more response windowsof a channel to receive a second random access message in response tothe first random access message, select one of the first random accessprocedure or a second random access procedure, where the selection isbased on a response window of the one or more response windows overwhich the second random access message is received, and establish aconnection with the base station based on the first random accessmessage, the second random access message, and the selected one of thefirst random access procedure or the second random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for transmitting a third random access message to the basestation based at least in part selecting the second random accessprocedure, the third random access message including a retransmission ofthe connection request, receiving a fourth random access message fromthe base station in response to the third random access message, thefourth random access message including a connection setup message inresponse to the connection request, and where establishing theconnection can be further based on the third random access message andthe fourth random access message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for initiating, following the random access preamble, afirst response window of the one or more response windows, monitoringthe first response window for receiving the second random access messageas part of the first random access procedure, initiating, following theconnection request, a second response window of the one or more responsewindows, and monitoring the second response window for receiving thesecond random access message as part of the second random accessprocedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first response window canbe based on a first configured timer and the second response window canbe based on a second configured timer.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first response window andthe second response window span different temporal durations.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first response window andthe second response window overlap during a temporal duration.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for initiating the response window of the one or moreresponse windows following the random access preamble, monitoring, priorto the connection request, the response window for receiving the secondrandom access message as part of the first random access procedure, andmonitoring, following the connection request, the response window forreceiving the second random access message as part of the first randomaccess procedure or the second random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for initiating the response window of the one or moreresponse windows following the connection request, and monitoring theresponse window for receiving the second random access message as partof the first random access procedure or the second random accessprocedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first random accessprocedure can be a two-step random access procedure and the secondrandom access procedure can be a four-step random access procedure.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a method of wireless communication at a UE. Themethod may include transmitting, to a base station, a first randomaccess message of a first random access procedure, the first randomaccess message including a random access preamble and a first redundancyversion of a connection request, receiving, in response to the firstrandom access message, a second random access message indicating aswitch from the first random access procedure to a second random accessprocedure, transmitting a third random access message in response to theindicated switch from the first random access procedure to the secondrandom access procedure, the third random access message including atleast one of a new data indicator or a second redundancy version of theconnection request, and receiving a fourth random access message fromthe base station in response to the third random access message, thefourth random access message including a connection setup message inresponse to the connection request.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a UE. The apparatus may include a processor, memory in electroniccommunication with the processor, and instructions stored in the memory.The instructions may be executable by the processor to cause theapparatus to transmit, to a base station, a first random access messageof a first random access procedure, the first random access messageincluding a random access preamble and a first redundancy version of aconnection request, receive, in response to the first random accessmessage, a second random access message indicating a switch from thefirst random access procedure to a second random access procedure,transmit a third random access message in response to the indicatedswitch from the first random access procedure to the second randomaccess procedure, the third random access message including at least oneof a new data indicator or a second redundancy version of the connectionrequest, and receive a fourth random access message from the basestation in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a UE. The apparatus may include means for transmitting, to a basestation, a first random access message of a first random accessprocedure, the first random access message including a random accesspreamble and a first redundancy version of a connection request,receiving, in response to the first random access message, a secondrandom access message indicating a switch from the first random accessprocedure to a second random access procedure, transmitting a thirdrandom access message in response to the indicated switch from the firstrandom access procedure to the second random access procedure, the thirdrandom access message including at least one of a new data indicator ora second redundancy version of the connection request, and receiving afourth random access message from the base station in response to thethird random access message, the fourth random access message includinga connection setup message in response to the connection request.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a non-transitory computer-readablemedium storing code for wireless communication at a UE. The code mayinclude instructions executable by a processor to transmit, to a basestation, a first random access message of a first random accessprocedure, the first random access message including a random accesspreamble and a first redundancy version of a connection request,receive, in response to the first random access message, a second randomaccess message indicating a switch from the first random accessprocedure to a second random access procedure, transmit a third randomaccess message in response to the indicated switch from the first randomaccess procedure to the second random access procedure, the third randomaccess message including at least one of a new data indicator or asecond redundancy version of the connection request, and receive afourth random access message from the base station in response to thethird random access message, the fourth random access message includinga connection setup message in response to the connection request.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for establishing a connection with the base station basedon the response to the connection request.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for receiving an indication of the second redundancyversion in the second random access message, and selecting the secondredundancy version of the connection request for the third random accessmessage based on the indication.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for identifying a subset of a set of supported redundancyversion identification values can be based on a received remainingminimum system information transmission.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for receiving a broadcast of system information prior totransmitting the first random access message, the system informationidentifying a set of supported redundancy versions for the first randomaccess message or the third random access message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for identifying the second redundancy version of theconnection request based on a standard configuration, and wheretransmitting the third random access message can be based on theidentifying.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the second random accessmessage includes a medium access control protocol data unit including atleast an uplink grant, a timing advance command, a network identifier,and a reserved bit.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first random accessprocedure can be a two-step random access procedure and the secondrandom access procedure can be a four-step random access procedure.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a method of wireless communication at a basestation. The method may include monitoring a channel for receiving, froma UE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request, determining a format of a second random accessmessage based on receiving the first random access message, where theformat of the second random access message indicates one of: the firstrandom access procedure or a second random access procedure,transmitting, to the UE, the second random access message in response tothe first random access message, and establishing a connection with thebase station based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a base station. The apparatus may include a processor, memory inelectronic communication with the processor, and instructions stored inthe memory. The instructions may be executable by the processor to causethe apparatus to monitor a channel for receiving, from a UE, a firstrandom access message of a first random access procedure, the firstrandom access message including a random access preamble and aconnection request, determine a format of a second random access messagebased on receiving the first random access message, where the format ofthe second random access message indicates one of: the first randomaccess procedure or a second random access procedure, transmit, to theUE, the second random access message in response to the first randomaccess message, and establish a connection with the base station basedon the first random access message, the second random access message,and the indicated one of the first random access procedure or the secondrandom access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a base station. The apparatus may include means for monitoring achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a connection request, determining a formatof a second random access message based on receiving the first randomaccess message, where the format of the second random access messageindicates one of: the first random access procedure or a second randomaccess procedure, transmitting, to the UE, the second random accessmessage in response to the first random access message, and establishinga connection with the base station based on the first random accessmessage, the second random access message, and the indicated one of thefirst random access procedure or the second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an non-transitory computer-readablemedium storing code for wireless communication at a base station. Thecode may include instructions executable by a processor to monitor achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a connection request, determine a format ofa second random access message based on receiving the first randomaccess message, where the format of the second random access messageindicates one of: the first random access procedure or a second randomaccess procedure, transmit, to the UE, the second random access messagein response to the first random access message, and establish aconnection with the base station based on the first random accessmessage, the second random access message, and the indicated one of thefirst random access procedure or the second random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for identifying the random access preamble and theconnection request based on the monitoring, configuring the secondrandom access message to include a random access response and aconnection setup message for the first random access procedure, andwhere determining the format of the second random access message can bebased on the configuring.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for identifying an absence of the connection request or aninability to decode a payload of the first random access message basedon the monitoring, configuring the second random access message toinclude a preamble index and an uplink grant indicating a switch fromthe first random access procedure to the second random access procedure,and where determining the format of the second random access message canbe based on the configuring.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for receiving, from the UE, a third random access messagebased on the switch from the first random access procedure to the secondrandom access procedure, the third random access message including aretransmission of the connection request, transmitting a fourth randomaccess message in response to the third random access message, thefourth random access message including a connection setup message inresponse to the connection request, and where establishing theconnection can be further based on the third random access message andthe fourth random access message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first random accessprocedure can be a two-step random access procedure and the secondrandom access procedure can be a four-step random access procedure.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a method of wireless communication at a basestation. The method may include monitoring a channel for receiving, froma UE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request, determining a payload of the first random accessmessage based on receiving the first random access message,transmitting, based on the determining, a second random access messagein response to the first random access message, where the second randomaccess message is associated with one of: the first random accessprocedure or a second random access procedure, and establishing aconnection with the base station based on the first random accessmessage, the second random access message, and the associated one of thefirst random access procedure or the second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a base station. The apparatus may include a processor, memory inelectronic communication with the processor, and instructions stored inthe memory. The instructions may be executable by the processor to causethe apparatus to monitor a channel for receiving, from a UE, a firstrandom access message of a first random access procedure, the firstrandom access message including a random access preamble and aconnection request, determine a payload of the first random accessmessage based on receiving the first random access message, transmit,based on the determining, a second random access message in response tothe first random access message, where the second random access messageis associated with one of: the first random access procedure or a secondrandom access procedure, and establish a connection with the basestation based on the first random access message, the second randomaccess message, and the associated one of the first random accessprocedure or the second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a base station. The apparatus may include means for monitoring achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a connection request, determining a payloadof the first random access message based on receiving the first randomaccess message, transmitting, based on the determining, a second randomaccess message in response to the first random access message, where thesecond random access message is associated with one of: the first randomaccess procedure or a second random access procedure, and establishing aconnection with the base station based on the first random accessmessage, the second random access message, and the associated one of thefirst random access procedure or the second random access procedure.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a non-transitory computer-readablemedium storing code for wireless communication at a base station. Thecode may include instructions executable by a processor to monitor achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a connection request, determine a payloadof the first random access message based on receiving the first randomaccess message, transmit, based on the determining, a second randomaccess message in response to the first random access message, where thesecond random access message is associated with one of: the first randomaccess procedure or a second random access procedure, and establish aconnection with the base station based on the first random accessmessage, the second random access message, and the associated one of thefirst random access procedure or the second random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for identifying the random access preamble and theconnection request based on the monitoring, and configuring the secondrandom access message to include a random access response and aconnection setup message for the first random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for identifying an absence of the connection request or aninability to decode the payload of the first random access message basedon the monitoring, and configuring the second random access message toinclude a preamble index and an uplink grant indicating a switch fromthe first random access procedure to the second random access procedure.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for multiplexing the second random access message with oneor more additional random access messages for a random access response.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for receiving, from the UE, a third random access messagebased on the switch from the first random access procedure to the secondrandom access procedure, the third random access message including aretransmission of the connection request, transmitting a fourth randomaccess message in response to the third random access message, thefourth random access message including a connection setup message inresponse to the connection request, and where establishing theconnection can be further based on the third random access message andthe fourth random access message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first random accessprocedure can be a two-step random access procedure and the secondrandom access procedure can be a four-step random access procedure.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a method of wireless communication at a basestation. The method may include monitoring a channel for receiving, froma UE, a first random access message of a first random access procedure,the first random access message including a random access preamble and afirst redundancy version of a connection request, identifying an absenceof the connection request or an inability to decode a payload of thefirst random access message based on the monitoring, transmitting, inresponse to the first random access message, a second random accessmessage indicating a switch from the first random access procedure to asecond random access procedure, receiving a third random access messagein response to the indicated switch from the first random accessprocedure to the second random access procedure, the third random accessmessage including at least one of a new data indicator or a secondredundancy version of the connection request, and transmitting a fourthrandom access message in response to the third random access message,the fourth random access message including a connection setup message inresponse to the connection request.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a base station. The apparatus may include a processor, memory inelectronic communication with the processor, and instructions stored inthe memory. The instructions may be executable by the processor to causethe apparatus to monitor a channel for receiving, from a UE, a firstrandom access message of a first random access procedure, the firstrandom access message including a random access preamble and a firstredundancy version of a connection request, identify an absence of theconnection request or an inability to decode a payload of the firstrandom access message based on the monitoring, transmit, in response tothe first random access message, a second random access messageindicating a switch from the first random access procedure to a secondrandom access procedure, receive a third random access message inresponse to the indicated switch from the first random access procedureto a second random access procedure, the third random access messageincluding at least one of a new data indicator or a second redundancyversion of the connection request, and transmit a fourth random accessmessage in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in an apparatus for wireless communicationat a base station. The apparatus may include means for monitoring achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a first redundancy version of a connectionrequest, identifying an absence of the connection request or aninability to decode a payload of the first random access message basedon the monitoring, transmitting, in response to the first random accessmessage, a second random access message indicating a switch from thefirst random access procedure to a second random access procedure,receiving a third random access message in response to the indicatedswitch from the first random access procedure to the second randomaccess procedure, the third random access message including at least oneof a new data indicator or a second redundancy version of the connectionrequest, and transmitting a fourth random access message in response tothe third random access message, the fourth random access messageincluding a connection setup message in response to the connectionrequest.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented in a non-transitory computer-readablemedium storing code for wireless communication at a base station. Thecode may include instructions executable by a processor to monitor achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a first redundancy version of a connectionrequest, identify an absence of the connection request or an inabilityto decode a payload of the first random access message based on themonitoring, transmit, in response to the first random access message, asecond random access message indicating a switch from the first randomaccess procedure to a second random access procedure, receive a thirdrandom access message in response to the indicated switch from the firstrandom access procedure to a second random access procedure, the thirdrandom access message including at least one of a new data indicator ora second redundancy version of the connection request, and transmit afourth random access message in response to the third random accessmessage, the fourth random access message including a connection setupmessage in response to the connection request.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for establishing a connection with the base station basedon the response to the connection request.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for configuring a subset of a set of supported redundancyversions, and indicating, in the second random access message, thesecond redundancy version of the connection request for the third randomaccess message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for transmitting, to the UE, a remaining system informationtransmission including the subset of the set of supported redundancyversions.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include operations, features, means, orinstructions for configuring a set of supported redundancy versions, andtransmitting a broadcast of system information prior to transmitting thefirst random access message, the system information identifying the setof supported redundancy versions for the first random access message orthe third random access message.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the second random accessmessage includes a medium access control protocol data unit including atleast an uplink grant, a timing advance command, a network identifier,and a reserved bit.

In some implementations, the method, apparatuses, and non-transitorycomputer-readable medium can include that the first random accessprocedure can be a two-step random access procedure and the secondrandom access procedure can be a four-step random access procedure.

Details of one or more implementations of the subject matter describedin this disclosure are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a wireless communications system thatsupports random access procedure fallback.

FIGS. 2A and 2B illustrate examples of wireless communications systemsthat support random access procedure fallback.

FIGS. 3A and 3B illustrate examples of transmission schemes that supportrandom access procedure fallback.

FIG. 4 illustrates an example of a process flow that supports randomaccess procedure fallback.

FIGS. 5A and 5B illustrate examples of transmission schemes that supportrandom access procedure fallback.

FIG. 6 illustrates an example of a process flow that supports randomaccess procedure fallback.

FIGS. 7 and 8 illustrate examples of process flows that support randomaccess procedure fallback.

FIGS. 9 and 10 illustrate block diagrams of example devices that supportrandom access procedure fallback.

FIG. 11 illustrates a block diagram of an example user equipment (UE)communications manager that supports random access procedure fallback.

FIG. 12 illustrates a diagram of an example system including a devicethat supports random access procedure fallback.

FIGS. 13 and 14 illustrate block diagrams of example devices thatsupport random access procedure fallback.

FIG. 15 illustrates a block diagram of an example base stationcommunications manager that supports random access procedure fallback.

FIG. 16 illustrates a diagram of an example system including a devicethat supports random access procedure fallback.

FIGS. 17-28 illustrate flowcharts illustrating example methods thatsupport random access procedure fallback.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The following description is directed to implementations for thepurposes of describing the innovative aspects of this disclosure.However, a person having ordinary skill in the art will readilyrecognize that the teachings herein can be applied in a multitude ofdifferent ways. The described implementations may be implemented in anydevice, system or network that is capable of transmitting and receivingradio frequency (RF) signals according to any of the 3GPP standards, orany of the Institute of Electrical and Electronics Engineers (IEEE)16.11 standards, or any of the IEEE 802.11 standards, the Bluetooth®standard, code division multiple access (CDMA), frequency divisionmultiple access (FDMA), time division multiple access (TDMA), GlobalSystem for Mobile communications (GSM), GSM/General Packet Radio Service(GPRS), Enhanced Data GSM Environment (EDGE), Terrestrial Trunked Radio(TETRA), Wideband-CDMA (W-CDMA), Evolution Data Optimized (EV-DO),1×EV-DO, EV-DO Rev A, EV-DO Rev B, High Speed Packet Access (HSPA), HighSpeed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access(HSUPA), Evolved High Speed Packet Access (HSPA+), Long Term Evolution(LTE), New Radio (NR), AMPS, or other known signals that are used tocommunicate within a wireless, cellular or internet of things (JOT)network, such as a system utilizing 3G, 4G or 5G, or furtherimplementations thereof, technology.

A user equipment (UE) and a base station (such as a eNodeB (eNB), anext-generation NodeB or giga-NodeB (either of which may be referred toas a gNB)) may establish a connection using a random access procedure.The random access procedure may include a series of handshake messagescarrying information that facilitates establishing the connectionbetween the UE and the base station. In some implementations, a randomaccess procedure may be a 2-step random access procedure, which mayreduce latency compared to other random access procedures that use agreater number of handshake messages (such as a 4-step random accessprocedure). The 2-step random access procedure may include a firstrandom access message (message A (msgA)) transmitted by the UE. The msgAtransmission may include a random access channel (RACH) preamble and aphysical uplink shared channel (PUSCH) payload that includes a message.In some implementations, the content of the message of the PUSCH payloadmay be based on a use case associated with the RACH, such as aconnection setup procedure (RRC IDLE to RRC CONNECTED), a connectionreestablishment procedure, a handover procedure, a buffer status report(BSR), or a request for system information (for example, other systeminformation). For example, the PUSCH payload may include one or more ofa connection request, such as a radio resource control (RRC) setuprequest (for a connection setup procedure), an RRC reestablishmentrequest (for a connection reestablishment procedure), and a UEidentification (ID) (for a handover procedure). In response, the basestation may transmit a second random access message (message B (msgB))that includes contents associated with a random access response and aconnection setup based on the use case associated with the RACH. Forexample, the msgB may include a response to one or more of theconnection request, the RRC reestablishment request, and the UE ID.

In some implementations, a base station may fail to receive or decodethe PUSCH payload of the msgA transmission due to signal interference ortraffic intensity on resources of the channel. For example, as part of acontention-based procedure the UE may transmit the preamble associatedwith msgA but fail to transmit the PUSCH payload due to a loss ofcontention (such as due to a signal gap between preamble and PUSCHpayload transmission). In some other examples, the PUSCH payload maysuffer from low signal to noise ratios due to interference or UEcollision where multiple UEs transmit the PUSCH payload on overlappingresources. The random access procedure may benefit from utilizing thereceived preamble information instead of performing msgA retransmission.

Accordingly, the UE may fallback from a 2-step random access procedureto a 4-step random access procedure when the 2-step random accessprocedure is unsuccessful. Because the base station and the UE maysupport multiple random access procedures (such as 2-step and 4-steprandom access procedures), the base station may configure transmissionof random access messages to enable the UE to differentiate betweenrandom access messages of different types of random access procedures(such as 2-step and 4-step random access procedures). In someimplementations, the configuration may include a formatting of the msgBtransmission to support multiple response types based on the msgAreception. In some other implementations, the configuration may includeindicating a new data indicator or one or more redundancy versionsassociated with a potential msg3 transmission, as part of the fallbackto the 4-step random access procedure. The msg3 transmission may includea PUSCH payload that includes a connection request, as well as a UEidentifier for contention resolution. Additionally, or alternatively,the UE may support one or more response windows for monitoring thechannel. The one or more response windows may be configured foridentifying at least one of a msgB transmission or a msg2 transmissionas part of the fallback to the 4-step random access procedure.

Particular implementations of the subject matter described in thisdisclosure may be implemented to realize one or more of the followingpotential advantages. For example, a 2-step random access procedure maybe performed to reduce delay in establishing communication between abase station and a UE. The 2-step random access procedure may includesupport for fallback communications to a 4-step random access procedure.Fallback communication may benefit random access procedure when signalattenuation or delays in signaling impede connection establishmentbetween a base station and a UE. As part of the fallback communication,a base station may receive and detect a preamble transmission withoutreceiving or decoding a PUSCH payload associated with a connectionrequest. The base station may utilize the received preamble forcontinued handshake exchange associated with the fallback to a 4-steprandom access procedure rather than performing retransmission. Byutilizing the received preamble, the base station may reduce signalingoverhead. In addition, a lower RACH latency may be achieved by thefallback procedure without the UE retransmitting the preamble in themsgA because the preamble has already been received by the base stationsuccessfully. In particular, the UE can be scheduled to perform msg3transmission by a msg2 reception, instead of waiting for a msgB responseexpiration and attempting another 2-step random access procedure.

FIG. 1 illustrates an example of a wireless communications system 100that supports random access procedure fallback. The wirelesscommunications system 100 includes base stations 105, UEs 115, and acore network 130. In some implementations, the wireless communicationssystem 100 may be an LTE network, an LTE-Advanced (LTE-A) network, anLTE-A Pro network, or an NR network. In some implementations, wirelesscommunications system 100 may support enhanced broadband communications,ultra-reliable (such as mission critical) communications, low latencycommunications, or communications with low-cost and low-complexitydevices.

Base stations 105 may wirelessly communicate with UEs 115 via one ormore base station antennas. Base stations 105 described herein mayinclude or may be referred to by those skilled in the art as a basetransceiver station, a radio base station, an access point, a radiotransceiver, a NodeB, an eNB, a next-generation NodeB or giga-NodeB(either of which may be referred to as a gNB), a Home NodeB, a HomeeNodeB, or some other suitable terminology. Wireless communicationssystem 100 may include base stations 105 of different types (such asmacro or small cell base stations). The UEs 115 described herein may beable to communicate with various types of base stations 105 and networkequipment including macro eNBs, small cell eNBs, gNBs, relay basestations, and the like.

Each base station 105 may be associated with a particular geographiccoverage area 110 in which communications with various UEs 115 issupported. Each base station 105 may provide communication coverage fora respective geographic coverage area 110 via communication links 125,and communication links 125 between a base station 105 and a UE 115 mayutilize one or more carriers. Communication links 125 shown in wirelesscommunications system 100 may include uplink transmissions from a UE 115to a base station 105, or downlink transmissions from a base station 105to a UE 115. Downlink transmissions also may be called forward linktransmissions while uplink transmissions also may be called reverse linktransmissions.

The geographic coverage area 110 for a base station 105 may be dividedinto sectors making up a portion of the geographic coverage area 110,and each sector may be associated with a cell. For example, each basestation 105 may provide communication coverage for a macro cell, a smallcell, a hot spot, or other types of cells, or various combinationsthereof. In some implementations, a base station 105 may be movable andtherefore provide communication coverage for a moving geographiccoverage area 110. In some implementations, different geographiccoverage areas 110 associated with different technologies may overlap,and overlapping geographic coverage areas 110 associated with differenttechnologies may be supported by the same base station 105 or bydifferent base stations 105. The wireless communications system 100 mayinclude, for example, a heterogeneous LTE/LTE-A/LTE-A Pro or NR networkin which different types of base stations 105 provide coverage forvarious geographic coverage areas 110.

The term “cell” refers to a logical communication entity used forcommunication with a base station 105 (such as over a carrier), and maybe associated with an identifier for distinguishing neighboring cells(such as a physical cell identifier (PCID), a virtual cell identifier(VCID)) operating via the same or a different carrier. In someimplementations, a carrier may support multiple cells, and differentcells may be configured according to different protocol types (such asmachine-type communication (MTC), narrowband IoT (NB-IoT), enhancedmobile broadband (eMBB), or others) that may provide access fordifferent types of devices. In some examples, the term “cell” may referto a portion of a geographic coverage area 110 (such as a sector) overwhich the logical entity operates.

UEs 115 may be dispersed throughout the wireless communications system100, and each UE 115 may be stationary or mobile. A UE 115 also may bereferred to as a mobile device, a wireless device, a remote device, ahandheld device, or a subscriber device, or some other suitableterminology, where the “device” also may be referred to as a unit, astation, a terminal, or a client. A UE 115 also may be a personalelectronic device such as a cellular phone, a personal digital assistant(PDA), a tablet computer, a laptop computer, or a personal computer. Insome implementations, a UE 115 also may refer to a wireless local loop(WLL) station, an IoT device, an Internet of Everything (IoE) device, oran MTC device, or the like, which may be implemented in various articlessuch as appliances, vehicles, meters, or the like.

Some UEs 115, such as MTC or IoT devices, may be low cost or lowcomplexity devices, and may provide for automated communication betweenmachines (such as via Machine-to-Machine (M2M) communication). M2Mcommunication or MTC may refer to data communication technologies thatallow devices to communicate with one another or a base station 105without human intervention. In some implementations, M2M communicationor MTC may include communications from devices that integrate sensors ormeters to measure or capture information and relay that information to acentral server or application program that can make use of theinformation or present the information to humans interacting with theprogram or application. Some UEs 115 may be designed to collectinformation or enable automated behavior of machines. Examples ofapplications for MTC devices include smart metering, inventorymonitoring, water level monitoring, equipment monitoring, healthcaremonitoring, wildlife monitoring, weather and geological eventmonitoring, fleet management and tracking, remote security sensing,physical access control, and transaction-based business charging.

Some UEs 115 may be configured to employ operating modes that reducepower consumption, such as half-duplex communications (such as a modethat supports one-way communication via transmission or reception, butnot transmission and reception simultaneously). In some implementationshalf-duplex communications may be performed at a reduced peak rate.Other power conservation techniques for UEs 115 include entering a powersaving “deep sleep” mode when not engaging in active communications, oroperating over a limited bandwidth (such as according to narrowbandcommunications). In some implementations, UEs 115 may be designed tosupport critical functions (such as mission critical functions), and awireless communications system 100 may be configured to provideultra-reliable communications for these functions.

In some examples, a UE 115 also may be able to communicate directly withother UEs 115 (such as using a peer-to-peer (P2P) or device-to-device(D2D) protocol). One or more of a group of UEs 115 utilizing D2Dcommunications may be within the geographic coverage area 110 of a basestation 105. Other UEs 115 in such a group may be outside the geographiccoverage area 110 of a base station 105, or be otherwise unable toreceive transmissions from a base station 105. In some examples, groupsof UEs 115 communicating via D2D communications may utilize aone-to-many (1:M) system in which each UE 115 transmits to every otherUE 115 in the group. In some examples, a base station 105 facilitatesthe scheduling of resources for D2D communications. In some other cases,D2D communications are carried out between UEs 115 without theinvolvement of a base station 105.

Base stations 105 may communicate with the core network 130 and with oneanother. For example, base stations 105 may interface with the corenetwork 130 through backhaul links 132 (such as via an S1, N2, N3, oranother interface). Base stations 105 may communicate with one anotherover backhaul links 134 (such as via an X2, Xn, or other interface)either directly (such as directly between base stations 105) orindirectly (such as via core network 130).

The core network 130 may provide user authentication, accessauthorization, tracking, Internet Protocol (IP) connectivity, and otheraccess, routing, or mobility functions. The core network 130 may be anevolved packet core (EPC), which may include at least one mobilitymanagement entity (MME), at least one serving gateway (S-GW), and atleast one Packet Data Network (PDN) gateway (P-GW). The MME may managenon-access stratum (such as control plane) functions such as mobility,authentication, and bearer management for UEs 115 served by basestations 105 associated with the EPC. User IP packets may be transferredthrough the S-GW, which itself may be connected to the P-GW. The P-GWmay provide IP address allocation as well as other functions. The P-GWmay be connected to the network operators IP services. The operators IPservices may include access to the Internet, Intranet(s), an IPMultimedia Subsystem (IMS), or a Packet-Switched (PS) Streaming Service.

At least some of the network devices, such as a base station 105, mayinclude subcomponents such as an access network entity, which may be anexample of an access node controller (ANC). Each access network entitymay communicate with UEs 115 through a number of other access networktransmission entities, which may be referred to as a radio head, a smartradio head, or a transmission/reception point (TRP). In someconfigurations, various functions of each access network entity or basestation 105 may be distributed across various network devices (such asradio heads and access network controllers) or consolidated into asingle network device (such as a base station 105).

Wireless communications system 100 may operate using one or morefrequency bands, typically in the range of 300 megahertz (MHz) to 300gigahertz (GHz). Generally, the region from 300 MHz to 3 GHz is known asthe ultra-high frequency (UHF) region or decimeter band, since thewavelengths range from approximately one decimeter to one meter inlength. UHF waves may be blocked or redirected by buildings andenvironmental features. However, the waves may penetrate structuressufficiently for a macro cell to provide service to UEs 115 locatedindoors. Transmission of UHF waves may be associated with smallerantennas and shorter range (such as less than 100 km) compared totransmission using the smaller frequencies and longer waves of the highfrequency (HF) or very high frequency (VHF) portion of the spectrumbelow 300 MHz.

Wireless communications system 100 also may operate in a super highfrequency (SHF) region using frequency bands from 3 GHz to 30 GHz, alsoknown as the centimeter band. The SHF region includes bands such as the5 GHz industrial, scientific, and medical (ISM) bands, which may be usedopportunistically by devices that may be capable of toleratinginterference from other users.

Wireless communications system 100 also may operate in an extremely highfrequency (EHF) region of the spectrum (such as from 30 GHz to 300 GHz),also known as the millimeter band. In some implementations, wirelesscommunications system 100 may support millimeter wave (mmW)communications between UEs 115 and base stations 105, and EHF antennasof the respective devices may be even smaller and more closely spacedthan UHF antennas. In some examples, this may facilitate use of antennaarrays within a UE 115. However, the propagation of EHF transmissionsmay be subject to even greater atmospheric attenuation and shorter rangethan SHF or UHF transmissions. Techniques disclosed herein may beemployed across transmissions that use one or more different frequencyregions, and designated use of bands across these frequency regions maydiffer by country or regulating body.

In some examples, wireless communications system 100 may utilize bothlicensed and unlicensed radio frequency spectrum bands. For example,wireless communications system 100 may employ License Assisted Access(LAA), LTE-Unlicensed (LTE-U) radio access technology, or NR technologyin an unlicensed band such as the 5 GHz ISM band. When operating inunlicensed radio frequency spectrum bands, wireless devices such as basestations 105 and UEs 115 may employ listen-before-talk (LBT) proceduresto ensure a frequency channel is clear before transmitting data. In someexamples, operations in unlicensed bands may be based on a carrieraggregation configuration in conjunction with component carriersoperating in a licensed band (such as LAA). Operations in unlicensedspectrum may include downlink transmissions, uplink transmissions,peer-to-peer transmissions, or a combination of these. Duplexing inunlicensed spectrum may be based on frequency division duplexing (FDD),time division duplexing (TDD), or a combination of both.

In some implementations, base station 105 or UE 115 may be equipped withmultiple antennas, which may be used to employ techniques such astransmit diversity, receive diversity, multiple-input multiple-output(MIMO) communications, or beamforming. For example, wirelesscommunications system 100 may use a transmission scheme between atransmitting device (such as a base station 105) and a receiving device(such as a UE 115), where the transmitting device is equipped withmultiple antennas and the receiving device is equipped with one or moreantennas. MIMO communications may employ multipath signal propagation toincrease the spectral efficiency by transmitting or receiving multiplesignals via different spatial layers, which may be referred to asspatial multiplexing. The multiple signals may, for example, betransmitted by the transmitting device via different antennas ordifferent combinations of antennas. Likewise, the multiple signals maybe received by the receiving device via different antennas or differentcombinations of antennas. Each of the multiple signals may be referredto as a separate spatial stream, and may carry bits associated with thesame data stream (such as the same codeword) or different data streams.Different spatial layers may be associated with different antenna portsused for channel measurement and reporting. MIMO techniques includesingle-user MIMO (SU-MIMO) where multiple spatial layers are transmittedto the same receiving device, and multiple-user MIMO (MU-MIMO) wheremultiple spatial layers are transmitted to multiple devices.

Beamforming, which also may be referred to as spatial filtering,directional transmission, or directional reception, is a signalprocessing technique that may be used at a transmitting device or areceiving device (such as a base station 105 or a UE 115) to shape orsteer an antenna beam (such as a transmit beam or receive beam) along aspatial path between the transmitting device and the receiving device.Beamforming may be achieved by combining the signals communicated viaantenna elements of an antenna array such that signals propagating atparticular orientations with respect to an antenna array experienceconstructive interference while others experience destructiveinterference. The adjustment of signals communicated via the antennaelements may include a transmitting device or a receiving deviceapplying certain amplitude and phase offsets to signals carried via eachof the antenna elements associated with the device. The adjustmentsassociated with each of the antenna elements may be defined by abeamforming weight set associated with a particular orientation (such aswith respect to the antenna array of the transmitting device orreceiving device, or with respect to some other orientation).

In one example, a base station 105 may use multiple antennas or antennaarrays to conduct beamforming operations for directional communicationswith a UE 115. For instance, some signals (such as synchronizationsignals, reference signals, beam selection signals, or other controlsignals) may be transmitted by a base station 105 multiple times indifferent directions, which may include a signal being transmittedaccording to different beamforming weight sets associated with differentdirections of transmission. Transmissions in different beam directionsmay be used to identify (such as by the base station 105 or a receivingdevice, such as a UE 115) a beam direction for subsequent transmissionor reception by the base station 105.

Some signals, such as data signals associated with a particularreceiving device, may be transmitted by a base station 105 in a singlebeam direction (such as a direction associated with the receivingdevice, such as a UE 115). In some implementations, the beam directionassociated with transmissions along a single beam direction may bedetermined based at least in in part on a signal that was transmitted indifferent beam directions. For example, a UE 115 may receive one or moreof the signals transmitted by the base station 105 in differentdirections, and the UE 115 may report to the base station 105 anindication of the signal it received with a highest signal quality, oran otherwise acceptable signal quality. Although these techniques aredescribed with reference to signals transmitted in one or moredirections by a base station 105, a UE 115 may employ similar techniquesfor transmitting signals multiple times in different directions (such asfor identifying a beam direction for subsequent transmission orreception by the UE 115), or transmitting a signal in a single direction(such as for transmitting data to a receiving device).

A receiving device (such as a UE 115, which may be an example of a mmWreceiving device) may try multiple receive beams when receiving varioussignals from the base station 105, such as synchronization signals,reference signals, beam selection signals, or other control signals. Forexample, a receiving device may try multiple receive directions byreceiving via different antenna subarrays, by processing receivedsignals according to different antenna subarrays, by receiving accordingto different receive beamforming weight sets applied to signals receivedat multiple antenna elements of an antenna array, or by processingreceived signals according to different receive beamforming weight setsapplied to signals received at multiple antenna elements of an antennaarray, any of which may be referred to as “listening” according todifferent receive beams or receive directions. In some implementations areceiving device may use a single receive beam to receive along a singlebeam direction (such as when receiving a data signal). The singlereceive beam may be aligned in a beam direction determined based atleast in part on listening according to different receive beamdirections (such as a beam direction determined to have a highest signalstrength, highest signal-to-noise ratio, or otherwise acceptable signalquality based at least in part on listening according to multiple beamdirections).

In some examples, the antennas of a base station 105 or UE 115 may belocated within one or more antenna arrays, which may support MIMOoperations, or transmit or receive beamforming. For example, one or morebase station antennas or antenna arrays may be co-located at an antennaassembly, such as an antenna tower. In some examples, antennas orantenna arrays associated with a base station 105 may be located indiverse geographic locations. A base station 105 may have an antennaarray with a number of rows and columns of antenna ports that the basestation 105 may use to support beamforming of communications with a UE115. Likewise, a UE 115 may have one or more antenna arrays that maysupport various MIMO or beamforming operations.

In some implementations, wireless communications system 100 may be apacket-based network that operate according to a layered protocol stack.In the user plane, communications at the bearer or Packet DataConvergence Protocol (PDCP) layer may be IP-based. A Radio Link Control(RLC) layer may perform packet segmentation and reassembly tocommunicate over logical channels. A Medium Access Control (MAC) layermay perform priority handling and multiplexing of logical channels intotransport channels. The media access control (MAC) layer also may usehybrid automatic repeat request (HARQ) to provide retransmission at theMAC layer to improve link efficiency. In the control plane, the RadioResource Control (RRC) protocol layer may provide establishment,configuration, and maintenance of an RRC connection between a UE 115 anda base station 105 or core network 130 supporting radio bearers for userplane data. At the Physical layer, transport channels may be mapped tophysical channels.

In some implementations, UEs 115 and base stations 105 may supportretransmissions of data to increase the likelihood that data is receivedsuccessfully. HARQ feedback is one technique of increasing thelikelihood that data is received correctly over a communication link125. HARQ may include a combination of error detection (such as using acyclic redundancy check (CRC)), forward error correction (FEC), andretransmission (such as automatic repeat request (ARQ)). HARQ mayimprove throughput at the MAC layer in poor radio conditions (such assignal-to-noise conditions). In some examples, a wireless device maysupport same-slot HARQ feedback, where the device may provide HARQfeedback in a specific slot for data received in a previous symbol inthe slot. In some other cases, the device may provide HARQ feedback in asubsequent slot, or according to some other time interval.

Time intervals in LTE or NR may be expressed in multiples of a basictime unit, which may, for example, refer to a sampling period ofT_(s)=1/30,720,000 seconds. Time intervals of a communications resourcemay be organized according to radio frames each having a duration of 10milliseconds (ms), where the frame period may be expressed asT_(f)=307,200 T_(s). The radio frames may be identified by a systemframe number (SFN) ranging from 0 to 1023. Each frame may include 10subframes numbered from 0 to 9, and each subframe may have a duration of1 ms. A subframe may be further divided into 2 slots each having aduration of 0.5 ms, and each slot may contain 6 or 7 modulation symbolperiods (such as depending on the length of the cyclic prefix prependedto each symbol period). Excluding the cyclic prefix, each symbol periodmay contain 2048 sampling periods. In some examples, a subframe may bethe smallest scheduling unit of the wireless communications system 100,and may be referred to as a transmission time interval (TTI). In someother cases, a smallest scheduling unit of the wireless communicationssystem 100 may be shorter than a subframe or may be dynamically selected(such as in bursts of shortened TTIs (sTTIs) or in selected componentcarriers using sTTIs).

In some wireless communications systems, a slot may further be dividedinto multiple mini-slots containing one or more symbols. In someinstances, a symbol of a mini-slot or a mini-slot may be the smallestunit of scheduling. Each symbol may vary in duration depending on thesubcarrier spacing or frequency band of operation, for example. Further,some wireless communications systems may implement slot aggregation inwhich multiple slots or mini-slots are aggregated together and used forcommunication between a UE 115 and a base station 105.

The term “carrier” refers to a set of radio frequency spectrum resourceshaving a defined physical layer structure for supporting communicationsover a communication link 125. For example, a carrier of a communicationlink 125 may include a portion of a radio frequency spectrum band thatis operated according to physical layer channels for a given radioaccess technology. Each physical layer channel may carry user data,control information, or other signaling. A carrier may be associatedwith a pre-defined frequency channel (such as an evolved universalmobile telecommunication system terrestrial radio access (E-UTRA)absolute radio frequency channel number (EARFCN)), and may be positionedaccording to a channel raster for discovery by UEs 115. Carriers may bedownlink or uplink (such as in an FDD mode), or be configured to carrydownlink and uplink communications (such as in a TDD mode). In someimplementations, signal waveforms transmitted over a carrier may be madeup of multiple sub-carriers (such as using multi-carrier modulation(MCM) techniques such as orthogonal frequency division multiplexing(OFDM) or discrete Fourier transform spread OFDM (DFT-S-OFDM)).

The organizational structure of the carriers may be different fordifferent radio access technologies (such as LTE, LTE-A, LTE-A Pro, NR).For example, communications over a carrier may be organized according toTTIs or slots, each of which may include user data as well as controlinformation or signaling to support decoding the user data. A carrieralso may include dedicated acquisition signaling (such assynchronization signals or system information, etc.) and controlsignaling that coordinates operation for the carrier. In someimplementations (such as in a carrier aggregation configuration), acarrier also may have acquisition signaling or control signaling thatcoordinates operations for other carriers.

Physical channels may be multiplexed on a carrier according to varioustechniques. A physical control channel and a physical data channel maybe multiplexed on a downlink carrier, for example, using time divisionmultiplexing (TDM) techniques, frequency division multiplexing (FDM)techniques, or hybrid TDM-FDM techniques. In some implementations,control information transmitted in a physical control channel may bedistributed between different control regions in a cascaded manner (suchas between a common control region or common search space and one ormore UE-specific control regions or UE-specific search spaces).

A carrier may be associated with a particular bandwidth of the radiofrequency spectrum, and In some implementations the carrier bandwidthmay be referred to as a “system bandwidth” of the carrier or thewireless communications system 100. For example, the carrier bandwidthmay be one of a number of predetermined bandwidths for carriers of aparticular radio access technology (such as 1.4, 3, 5, 10, 15, 20, 40,or 80 MHz). In some implementations, each served UE 115 may beconfigured for operating over portions or all of the carrier bandwidth.In some other implementations, some UEs 115 may be configured foroperation using a narrowband protocol type that is associated with apredefined portion or range (such as set of subcarriers or RBs) within acarrier (such as “in-band” deployment of a narrowband protocol type).

In a system employing MCM techniques, a resource element may consist ofone symbol period (such as a duration of one modulation symbol) and onesubcarrier, where the symbol period and subcarrier spacing are inverselyrelated. The number of bits carried by each resource element may dependon the modulation scheme (such as the order of the modulation scheme).Thus, the more resource elements that a UE 115 receives and the higherthe order of the modulation scheme, the higher the data rate may be forthe UE 115. In MIMO systems, a wireless communications resource mayrefer to a combination of a radio frequency spectrum resource, a timeresource, and a spatial resource (such as spatial layers), and the useof multiple spatial layers may further increase the data rate forcommunications with a UE 115.

Devices of the wireless communications system 100 (such as base stations105 or UEs 115) may have a hardware configuration that supportscommunications over a particular carrier bandwidth, or may beconfigurable to support communications over one of a set of carrierbandwidths. In some implementations, the wireless communications system100 may include base stations 105 or UEs 115 that support simultaneouscommunications via carriers associated with more than one differentcarrier bandwidth.

Wireless communications system 100 may support communication with a UE115 on multiple cells or carriers, a feature which may be referred to ascarrier aggregation or multi-carrier operation. A UE 115 may beconfigured with multiple downlink component carriers and one or moreuplink component carriers according to a carrier aggregationconfiguration. Carrier aggregation may be used with both FDD and TDDcomponent carriers.

In some implementations, wireless communications system 100 may utilizeenhanced component carriers (eCCs). An eCC may be characterized by oneor more features including wider carrier or frequency channel bandwidth,shorter symbol duration, shorter TTI duration, or modified controlchannel configuration. In some examples, an eCC may be associated with acarrier aggregation configuration or a dual connectivity configuration(such as when multiple serving cells have a suboptimal or non-idealbackhaul link). An eCC also may be configured for use in unlicensedspectrum or shared spectrum (such as where more than one operator isallowed to use the spectrum). An eCC characterized by wide carrierbandwidth may include one or more segments that may be utilized by UEs115 that are not capable of monitoring the whole carrier bandwidth orare otherwise configured to use a limited carrier bandwidth (such as toconserve power).

In some implementations, an eCC may utilize a different symbol durationthan other component carriers, which may include use of a reduced symbolduration as compared with symbol durations of the other componentcarriers. A shorter symbol duration may be associated with increasedspacing between adjacent subcarriers. A device, such as a UE 115 or basestation 105, utilizing eCCs may transmit wideband signals (such asaccording to frequency channel or carrier bandwidths of 20, 40, 60, 80MHz, etc.) at reduced symbol durations (such as 16.67 microseconds). ATTI in eCC may consist of one or multiple symbol periods. In someexamples, the TTI duration (that is, the number of symbol periods in aTTI) may be variable.

Wireless communications system 100 may be an NR system that may utilizeany combination of licensed, shared, and unlicensed spectrum bands,among others. The flexibility of eCC symbol duration and subcarrierspacing may allow for the use of eCC across multiple spectrums. In someimplementations, NR shared spectrum may increase spectrum utilizationand spectral efficiency, specifically through dynamic vertical (such asacross the frequency domain) and horizontal (such as across the timedomain) sharing of resources.

A base station may perform a connection procedure (such as an RRCprocedure, a cell acquisition procedure, a random access procedure, anRRC connection procedure, an RRC configuration procedure) with a UE 115.For example, a base station 105 and a UE 115 may perform a random accessprocedure to establish a connection. In some other implementations, abase station 105 and a UE 115 may perform a random access procedure tore-establish a connection after a connection failure (such as aradio-link failure) with the base station 105, or to establish aconnection for handover to another base station, or the like. As part ofthe random access procedure, a UE 115 may transmit a RACH preamble. Thismay enable the base station 105 to distinguish between multiple UEs 115attempting to access the wireless communications system 100simultaneously.

The base station 105 may respond with a random access response thatprovides an uplink resource grant, a timing advance, and a temporarycell-specific radio network temporary identifier (C-RNTI). The UE 115may transmit an RRC connection request along with a temporary mobilesubscriber identity (TMSI) (if the UE 115 has previously been connectedto the same wireless network) or a random identifier. The RRC connectionrequest also may indicate the reason the UE 115 is connecting to thenetwork (emergency, signaling, or data exchange, among other examples).The base station 105 may respond to the connection request with acontention resolution message addressed to the UE 115, which may providea new C-RNTI. If the UE 115 receives a contention resolution messagewith the correct identification, it may proceed with RRC connectionsetup. If the UE 115 does not, however, receive a contention resolutionmessage (if there is a conflict with another UE 115) the UE 115 mayrepeat the RACH process by transmitting a new RACH preamble. Asdescribed, the exchange of messages between the UE 115 and the basestation 105 for random access may be referred to as a 4-step randomaccess procedure.

In some other implementations, a 2-step random access procedure may beperformed for random access. UEs 115 that operate in licensed orunlicensed spectrum within the wireless communications system 100 mayparticipate in a 2-step random access procedure to reduce delay inestablishing communication with a base station 105 (such as compared toa 4-step random access procedure). In some implementations, the 2-steprandom access procedure may operate regardless of whether a UE 115 has avalid timing advance parameter. For example, a UE 115 may use a validtiming advance parameter to coordinate the timing of its transmissionsto a base station 105 (to account for propagation delay) and may receivethe valid timing advance parameter as part of the 2-step random accessprocedure. Additionally, the 2-step random access procedure may beapplicable to any cell size, may work regardless of whether the randomaccess procedure is contention-based or contention-free, and may combinemultiple random access messages from a 4-step random access procedure.

For example, a first random access message (msgA), sent from a UE 115 toa base station 105 may combine the contents of a random access message 1(msg1) and message 3 (msg3) from 4-step RACH. Additionally, msgA mayconsist of a RACH preamble and a PUSCH carrying a payload with thecontents of the message (equivalent to msg3), where the preamble and thepayload may be transmitted on separate waveforms. In someimplementations, the base station 105 may transmit a downlink controlchannel (such as a physical downlink control channel (PDCCH)) and acorresponding second random access message (such as a msgB) to the UE115, where msgB may combine the equivalent contents of a random accessmessage 2 and message 4 from 4-step RACH. In some implementations of2-step RACH, a base station 105 may transmit msgB using either broadcastmethods (for example, targeting multiple UEs 115) or unicast methods(for example, targeting a specific UE 115).

In some implementations, a 2-step random access procedure may includesupport for fallback communications to a 4-step random access procedure.Fallback communication may benefit random access procedure when signalattenuation or delays in signaling impede connection establishmentbetween a base station 105 and a UE 115. For example, a UE 115 maytransmit a first random access message (such as a msgA) to a basestation 105 as part of a 2-step random access procedure. The basestation 105 may receive and detect the preamble included in the msgA,but may fail to decode the included PUSCH payload. In some otherimplementations, the random access procedure may be contention-based.The UE 115 may transmit the preamble associated with the msgA, but mayexperience intermittence in transmitting the PUSCH payload based on datatraffic on the channel. The base station 105 may receive and detect thepreamble without receiving the PUSCH payload associated with the randomaccess procedure. It may be advantageous for the base station 105 toutilize the received preamble for continued handshake message exchangeassociated with a fallback to a 4-step random access procedure, ratherthan performing retransmission of the msgA associated with the 2-steprandom access procedure. Specifically, by utilizing the receivedpreamble, the base station 105 may reduce signaling overhead.

As described herein, In some implementations, the base station 105 mayconfigure different formats for a second random access message (such asa msgB). The different formats may correspond to response types of themsgB transmission and include message payloads based on the contents ofthe received msgA. For example, the base station 105 may configure afirst response type for 2-step random access procedure. In some otherimplementations, the base station 105 may configure a second responsetype for fallback from the 2-step to a 4-step random access procedure.In some other implementations, the base station 105 may provide distinctresponse messages based on the received payload associated with the msgAtransmission. Specifically, the base station 105 may provide a msg2response (for example, as part of a 4-step random access procedure)following unsuccessful msgA reception and decoding, and as part of afallback procedure. Alternatively, the base station 105 may provide amsgB response following a successful msgA response for 2-step randomaccess procedure. In some other implementations, the base station 105may configure a set of redundancy versions for fallback response basedon an unsuccessful msgA reception or decoding. The base station 105 mayindicate a new data indication or a redundancy version as part of anuplink grant included in a msg2 response and based on the fallback to a4-step random access procedure.

A UE 115 may differentiate between the configured response types of areceived msgB response. Additionally, or alternatively, the UE 115 maymonitor the channel for reception according to one or more responsewindows. Each of the one or more response windows may be configured formsgB reception for 2-step random access procedure or msg2 reception forfallback from a 2-step to a 4-step random access procedure. Based on afallback from a 2-step to a 4-step random access procedure, the UE 115may retransmit the PUSCH payload of msgA in a third random accessmessage (such as a msg3 for 4-step RACH). Following the msg3transmission the UE 115 may receive a fourth random access message (suchas a message 4 (msg4)) including a connection setup message in responseto the connection request. As a result, UEs 115 may be capable ofestablishing a connection with one or more base stations 105 viafallback methods to a 4-step random access procedure in the circumstanceof transmission failure associated with a 2-step random accessprocedure.

FIG. 2A illustrates an example of a wireless communications system 200-athat supports random access procedure fallback. The wirelesscommunications system 200-a may include a base station 105-a and a UE115-a, which may be examples of the corresponding devices described withreference to FIG. 1. In some implementations, the wirelesscommunications system 200-a may implement aspects of the wirelesscommunications system 100. For example, the base station 105-a and theUE 115-a may perform connectivity establishment via random accessprocedure.

The base station 105-a may perform a connection procedure (such as anRRC procedure, such as a cell acquisition procedure, a random accessprocedure, an RRC connection procedure, an RRC configuration procedure)with the UE 115-a. For example, base station 105-a and UE 115-a mayperform a random access procedure to establish a connection for wired orwireless communication. In some other implementations, base station105-a and UE 115-a may perform a random access procedure to re-establisha connection after a connection failure (such as a radio-link failure)with the base station 105-a, or to establish a connection for handoverto another base station, or the like. The base station 105-a and the UE115-a also may support multiple radio access technologies including 4Gsystems such as LTE systems, LTE-A systems, or LTE-A Pro systems, and 5Gsystems which may be referred to as NR systems.

The connection procedure (such as random access procedure) between thebase station 105-a and the UE 115-a may correspond to, for example, atleast one of the above example radio access technologies. By way ofexample, in FIG. 2A, a random access procedure may be related to 4Gsystems and may be referred to as a 4-step random access procedure. Aspart of the 4-step random access procedure, base station 105-a and UE115-a may transmit one or more messages (handshake messages), such as arandom access message 205 (also referred to herein as msg1), a randomaccess message 210 (also referred to herein as msg2), a random accessmessage 215 (also referred to herein as msg3), and a random accessmessage 220 (also referred to herein as msg4).

The UE 115-a may initiate the random access procedure by transmittingthe random access message 205, which may include a preamble (alsoreferred to a RACH preamble, a physical random access channel (PRACH)preamble, or a sequence) that may carry information, such as a UEidentifier. The purpose of the preamble transmission may be to providean indication to the base station 105-a presence of a random accessattempt, and to allow the base station 105-a to determine a delay (suchas a timing delay) between the base station 105-a and the UE 115-a. TheUE 115-a may transmit the random access message 205 to the base station105-a on a PRACH, for example.

The preamble of the random access message 205 may, in someimplementations, be defined by a preamble sequence and a cyclic prefix.A preamble sequence may be defined based in part on a Zadoff-Chusequence. The UE 115-a may additionally, or alternatively, use a guardperiod to handle timing uncertainty of the random access message 205transmission. For example, before beginning the random access procedure,the UE 115-a may obtain downlink synchronization with the base station105-a based in part on a cell-search procedure. However, because the UE115-a has not yet obtained uplink synchronization with the base station105-a, there may be an uncertainty in uplink timing due to the locationof the UE 115-a in the cell (such as geographic coverage area of basestation 105-a) not being known. In some implementations, the uncertaintyin uplink timing may be based in part on a dimension (such as a size orarea) of the cell. Therefore, including a cyclic prefix to the randomaccess message 205 may be beneficial, in some implementations, forhandling the uncertainty in uplink timing.

Per cell, there may be a number of preamble sequences (such as 64preamble sequences). The UE 115-a may select a preamble sequence from aset of sequences in a cell (for example, geographic coverage area ofbase station 105-a) based in part on a random selection. In someimplementations, the UE 115-a may select a preamble sequence based inpart on an amount of traffic that the UE 115-a has for transmission onan uplink shared channel (UL-SCH). From the preamble sequence that theUE 115-a selected, the base station 105-a may determine the amount ofuplink resources to be granted to the UE 115-a.

Some implementations of a random access procedure may becontention-based or contention-free. When performing a contention-basedrandom access procedure, the UE 115-a may select a preamble sequencefrom a set of sequences. That is, as long as other UEs (not shown) arenot performing a random access attempt using the same sequence at a sametemporal instance, no collisions will occur and the random accessattempt may be detected by the base station 105-a. If the UE 115-a isperforming a contention-free random access attempt, for example, for ahandover to a new cell, the preamble sequence to use may be explicitlysignaled (in control information) by the base station 105-a. To avoidcollisions or interference, the base station 105-a may select acontention-free preamble sequence from sequences not associated with thecontention-based random access attempt.

Upon receiving the random access message 205, the base station 105-a mayrespond appropriately with a random access message 210. For example, thebase station 105-a may transmit the random access message 210 to the UE115-a on a downlink shared channel (DL-SCH) or a PDCCH. In someimplementations, the random access message 210 may have a same or adifferent configuration (format) compared to the random access message205. The random access message 210 may carry information for the UE115-a, where the information is determined by the base station 105-abased in part on information carried in the random access message 205.For example, the information in the random access message 210 mayinclude an index of a preamble sequence detected and for which theresponse is valid, a timing advance parameter determined based in parton the preamble sequence detected, a scheduling grant indicating timeand frequency resources for the UE 115-a to use for transmission of anext random access message transmission by the UE 115-a, or a networkidentifier (such as a random access RNTI (RA-RNTI)) for furthercommunication with the UE 115-a, or the like.

In some implementations, if the base station 105-a detects multiplerandom access attempts (from the UE 115-a and other UEs (not shown)),the base station 105-a may combine individual response messages ofmultiple UEs in a single transmission (such as a MAC protocol data unit(PDU)), as described herein. As such, the random access message 210 maybe scheduled on a PDCCH using an identity reserved for random accessmessaging, for example, an RA-RNTI. The UE 115-a (and additional UEs(not shown)) may monitor the PDCCH to detect and receive a random accessmessage (such as the random access message 210). In someimplementations, the UE 115-a may monitor the PDCCH for a random accessmessage transmission from the base station 105-a during a random accessresponse window, which may be fixed or variable in size. For example, ifthe UE 115-a does not detect and receive a random access messagetransmission from the base station 105-a, the random access attempt maybe as associated with a failure and the random access procedure in FIG.2A may repeat. However, in the subsequent attempt, the random accessresponse window may be adjusted (such as increased or decreased inlength (duration)).

Once the UE 115-a successfully receives the random access message 210,the UE 115-a may obtain uplink synchronization with the base station105-a. In some implementations, before data transmission from the UE115-a, a unique identifier within the cell (such as a C-RNTI) may beassigned to the UE 115-a. In some implementations, depending on a state(such as a connected state, ideal state) of the UE 115-a there may be aneed for additional message (such as a connection request message)exchange for setting up the connection between the base station 105-aand the UE 115-a. The UE 115-a may transmit any additional messages, forexample, the random access message 215 to the base station 105-a usingthe UL-SCH resources (or PUSCH resources) assigned in the random accessmessage 210. The random access message 210 may include a UE identifierfor contention resolution. If the UE 115-a is in a connected state, forexample, the UE identifier may be a C-RNTI. Otherwise, the UE identifiermay be specific to the UE 115-a.

The base station 105-a may receive the random access message 215 and mayrespond properly, for example, by transmitting the random access message220, which may be a contention resolution message. When multiple UEs(including UE 115-a) are simultaneously performing random accessattempts using a same preamble sequence, the multiple UEs may listen fora same response message (such as a random access message 220). Each UE(including UE 115-a) may receive the random access message 220 andcompare an identifier (such as a network identifier) in the randomaccess message 220 to the identifier specified in the random accessmessage 215. When the identifiers match, the corresponding UE (such asUE 115-a) may declare the random access procedure successful. UEs thatdo not identify a match between the identifiers are considered to havefailed the random access procedure and may repeat the random accessprocedure with the base station 105-a. As a result of the connectionprocedure, the base station 105-a and the UE 115-a may establish aconnection for wired or wireless communication.

Although the connection procedure (such as the random access procedure)in FIG. 2A may be effective for facilitating random access for the UE115-a, there may be unnecessary latencies associated with thisprocedure. For example, latencies related to contention-based protocolof random access messaging may exhaust additional resources of the UE115-a. The techniques described herein may provide efficacy to the UE115-a by reducing or eliminating latencies associated with processesrelated to initial channel access (such as minimizing delay due tocontention-based protocol for RACH messaging), and more specifically toconstructing a random access message (msgB) transmission for a 2-steprandom access procedure that combines aspects of random access messages(msg2,4) associated with a 4-step random access procedure. The describedtechniques also provide for enabling a UE (such as the UE 115-a) and abase station (such as the base station 105-a) to support fallbackcommunication from 2-step to 4-step random access procedures.

FIG. 2B illustrates an example of a wireless communications system 200-bthat supports random access procedure fallback. The wirelesscommunications system 200-b may include a base station 105-b and a UE115-b, which may be examples of the corresponding devices described withreference to FIGS. 1 and 2A. In some implementations, the wirelesscommunications system 200-b may implement aspects of the wirelesscommunications system 100 or the wireless communications system 200-a.For example, the base station 105-b may perform connectivityestablishment (such as a random access procedure) with the UE 115-b toestablish a connection, or the like. The base station 105-b and the UE115-b may, as explained with reference to FIG. 2A, support multipleradio access technologies including 4G systems such as LTE systems,LTE-A systems, or LTE-A Pro systems, and 5G systems which may bereferred to as NR systems. The connection procedure (random accessprocedure) between the base station 105-b and the UE 115-b maycorrespond to, for example, at least one of these example radio accesstechnologies.

For example, in FIG. 2B, the random access procedure may be related to5G systems and may be referred to as a 2-step random access procedure.As part of a 2-step random access procedure, to decrease latenciesrelated to contention-based procedure, the base station 105-b and the UE115-b may exchange fewer messages (handshake messages) compared to a4-step random access procedure. For example, the UE 115-b may transmit asingle message, such as a random access message 225 (also referred toherein as msgA), and the base station 105-b may transmit a singlemessage, such as a random access response message 230 (also referred toherein as msgB) in response to the random access message 225. The randomaccess message 225 (msgA) may combine parts of msgs1,3 of a 4-steprandom access procedure, while the random access response message 230(msgB) may combine aspects of msgs2,4 of the 4-step random accessprocedure. When supporting both 2-step and 4-step random accessprocedures, it may be important for the UE 115-b to be configured todifferentiate between random access message formats (such as msgA, msgB,and msgs1,2,3,4).

When supporting both 2-step and 4-step random access procedures, in someimplementations, base station 105-b and UE 115-b may commence with onerandom access procedure (such as a 2-step random access procedure) andmay, in some implementations, fallback to another random accessprocedure (such as a 4-step random access procedure). In someimplementations, the base station 105-b may select or assign a priorityto a random access procedure based in part on a parameter (such as atraffic type or a network load). For example, the base station 105-b maybe configured to use a 4-step random access procedure over a 2-steprandom access procedure in certain scenarios (such as based on a trafficload) to reduce an overhead. For example, because the base station 105-bmay have to provision for msgA resources for 2-step RACH, which mayinvolve a larger overhead compared to msg1 transmissions with 4-stepRACH, the base station 105-b may select a 4-step random access procedureto maintain a lower overhead or to reduce an overhead in scenariosassociated with high traffic loads. Additionally, or alternatively, thebase station 105-b may select a random access procedure based in part ona UE 115-b capability to support the random access procedure. Forexample, if the UE 115-b supports both 2-step and 4-step random accessprocedures, the base station 105-b may select the 2-step or the 4-steprandom access procedure to commence the initial access procedure.Otherwise, the base station 105-b may select the random access proceduresupported by the UE 115-b.

In some implementations, UE 115-b may transmit the random access message225 to the base station 105-b. The random access message 225 may includea preamble and a PUSCH carrying a payload, where information in therandom access message 225 (msgA) may include the equivalent contents oraspects of random access message 215 (msg3 of a 4-step random accessprocedure). An advantage of the 2-step random access procedure relativeto the 4-step random access procedure is that the UE 115-b may becapable of transmitting data (such as a payload in PUSCH) to the basestation 105-b without requiring a connected state for data transmission.The base station 105-b may monitor a PUSCH for a random access preambleor a payload of the random access message 225. In some implementations,the payload may carry a connection request.

In some specific implementations, the base station 105-b may determinean absence of the random access preamble or the payload of the randomaccess message 225 based in part on the monitoring. For random accessprocedures in the unlicensed spectrum, the absence may be due to a gapbetween the preamble transmission and PUSCH transmission carrying apayload and UE may perform LBT between the preamble transmission and thePUSCH transmission. For example, the UE 115-b may perform carriersensing during the gap as part of an LBT procedure and fail to acquireresources of the contention for transmission of the payload. In someother implementations, the base station 105-b may fail to decode therandom access preamble or the payload of the random access message 225due to signal attenuation or interference. Absence of or an inability todecode one or both of the random access preamble or the payload of therandom access message 225 may result in a random access procedurefailure for the 2-step random access procedure.

For example, the base station 105-b may receive and detect the preambleincluded in the random access message 225 but may fail to receive ordecode the PUSCH transmission carrying the payload. It may beadvantageous for the base station 105-b to utilize the received preamblefor continued handshake message exchange associated with a fallback to4-step random access procedure, rather than performing retransmission ofthe random access message 225 associated with the 2-step random accessprocedure. Specifically, by utilizing the received preamble, the basestation 105-b may reduce signaling overhead and potential for continuedsignaling corruption or interference.

After receiving the random access message 225, the base station 105-bmay construct and transmit the random access response message 230 to theUE 115-b. In some implementations, the random access response message230 may be based on the received and decoded contents of the randomaccess message 225, including at least the preamble transmission and thePUSCH transmission carrying the payload. For example, the base station105-b may transmit the random access response message 230 to the UE115-b on a DL-SCH, PDSCH, or a PDCCH. The random access response message230 may include at least one of a network identifier of the UE 115-b, atiming advance parameter, or a backoff indication for the UE 115-b.

As described herein, in some implementations, the base station 105-b mayconfigure different formats for the random access response message 230.The different formats may correspond to response types of the randomaccess response message 230, and include message payloads based on thecontents of the received random access message 225 (for example, whetherthe payload was received and decoded). For example, the base station 105may configure a first response type for 2-step random access procedure,as described. In some other implementations, the base station 105-b mayconfigure a second response type for fallback from the 2-step to a4-step random access procedure, as described with reference to FIG. 2A.

In some other implementations, the base station 105 may provide adistinct response message based on the fallback from the 2-step randomaccess procedure to the 4-step random access procedure. For example, thebase station 105-b may transmit an alternative response message(including contents or aspects of random access message 210) rather thanthe random access response message 230. Additionally, or alternatively,the base station 105-b may configure a set of redundancy versions forfallback response. The base station 105-b may indicate a new dataindicator or a redundancy version as part of an uplink grant included ina random access response message (random access message 210) and basedon the fallback to a 4-step random access procedure.

The UE 115-b may monitor one or more response windows associated withthe present random access procedure (such as 2-step random accessprocedure) or a fallback to an alternative random access procedure (suchas 4-step random access procedure). For example, the UE 115-b maymonitor at least one response window for receiving the random accessresponse message 230. Additionally, or alternatively, the UE 115-b maymonitor at least one response window for receiving an alternative randomaccess response (random access message 210). In some otherimplementations, the UE 115-b may differentiate between the configuredresponse types of a received random access response message 230.

As part of a fallback communication from a 2-step to a 4-step randomaccess procedure, the UE 115-b may retransmit the PUSCH payload in athird random access message (contents or aspects of random accessmessage 215 (msg3)). Following the retransmission, the UE 115-b mayreceive a fourth random access message (contents or aspects of randomaccess message 220 (msg4)) as part of a random access response. Theresponse may include at least one of a network identifier of the UE115-b, a timing advance parameter, or a backoff indication for the UE115-b. The backoff indication may include a timing backoff indication ora random access procedure backoff indication, or both. The timingbackoff indication may be associated with a timing of the random accessprocedure and the random access procedure backoff indication may beassociated with the fallback from the 2-step random access procedure toan alternative random access procedure (such as 4-step random accessprocedure).

The techniques described herein for 2-step random access procedure mayreduce or eliminate latencies associated with processes related toinitial channel access and connectivity establishment between the UE115-b and the base station 105-b. Specifically, the UE 115-b mayconstruct a random access message (random access message 225) thatcombines aspects of random access messages (msg1,3) including a preambleand PUSCH payload for a connectivity request. Similarly, the basestation 105-b may construct a random access message (random accessresponse message 230) that combines aspects of random access messages(msg2, 4) configured for single message transmission. The describedtechniques also may provide support and configuration for fallback to a4-step RACH due to reception or decoding failure at the base station105-b. As part of the configuration, the base station 105-b may utilizereceived message contents for fallback procedure and reduce signalingoverhead. The described techniques also provide for enabling a UE 115 todifferentiate random access messages related to different types ofrandom access procedures (such as a 2-step random access procedure,4-step random access procedure) and act appropriately.

FIGS. 3A and 3B illustrate examples of transmission schemes 300-a and300-b that support random access procedure fallback. The transmissionschemes 300-a and 300-b may be implemented for connectivityestablishment between a UE and a base station, as described withreference to FIGS. 1, 2A, and 2B. The transmission schemes 300-a and300-b may include a configuration for random access response message(msgB) transmission by a base station, and a configured channelmonitoring for reception by the UE.

As described herein, the transmission schemes 300-a and 300-b maysupport connectivity establishment between a base station and a UEaccording to both a 2-step random access procedure and fallback from a2-step to a 4-step random access procedure. When supporting both 2-stepand 4-step random access procedures, in some implementations, the basestation and the UE may commence with one random access procedure (suchas 2-step random access procedure) and may, in some implementations,fallback to another random access procedure (such as 4-step randomaccess procedure). Additionally, the base station may support differentresponse formats for random access response message (msgB) transmissionaccording to the configuration. The response formats may correspond todifferent response types and may be based on detecting a PUSCHtransmission associated with a prior random access message (msgA)reception at the base station.

For example, the base station may configure a first payload type of therandom access response message (msgB) based on a reception and decodingof a PUSCH transmission, including a payload. The first response typemay include an identifier indication (for example, a name) associatedwith the corresponding UE and may be based on the received payload ofthe random access message (msgA). In some other implementations, thebase station may configure a second payload type of the random accessresponse message (msgB) based on a failure to receive or decode thepreceding transmission associated with the random access procedure. Thesecond response type may include a preamble index identifier and anuplink grant for fallback transmission associated with a switch to a4-step random access procedure.

Based on the one or more transmissions associated with the random accessmessage (msgA), the UE may monitor the channel for reception of theconfigured random access response message (msgB). The UE may monitor thechannel according to a response window (such as a response window 315 ora response window 320) and the response window may be configuredaccording to a timer. In some implementations, the timer may be based onone or more transmission instances by the UE in association with therandom access procedure. For example, in FIG. 3A, the UE may sense thechannel for initiating random access message (msgA) transmission. Basedon an identification of available channel resources, the UE may transmita preamble transmission 305-a carrying information, such as a UEidentifier. The UE may subsequently transmit a PUSCH transmission 310-acarrying a payload including a connection request. Based on the responsewindow 315, the UE may initiate a response window following the PUSCHoccasion and monitor channel resources for a random access responsemessage (msgB).

In some other implementations, such as FIG. 3B, the UE may sense thechannel for initiating random access message (msgA) transmission. Basedon an identification of available channel resources, the UE may transmita preamble transmission 305-b carrying information. The UE may initiatea response window 320 following the preamble transmission and monitorchannel resources for a random access response message (msgB). Bymonitoring the channel following the preamble transmission 305-b, the UEmay receive a msgB configured for a fallback response prior to PUSCHtransmission. As such, in some implementations, a msgB may be configuredfor both successful random access response (RAR) and fallback RAR for a2-step random access procedure. Such reception may enhance communicationbetween the base station and the UE, particularly when a PUSCHtransmission 310-b may not be proximal to the preamble transmission305-b (such as when there is a significant timing gap between the PUSCHtransmission 310-b and the preamble transmission 305-b).

The UE may transmit the PUSCH transmission 310-b carrying a payload,including a connection request. Following the PUSCH transmission, the UEmay continue to monitor the channel as part of the initiated responsewindow 320, as shown. The response window 320 may exclude the temporalduration corresponding to the PUSCH transmission 310-b, in order toavoid signaling interference at the UE.

FIG. 4 illustrates an example of a process flow 400 that supports randomaccess procedure fallback. The process flow 400 may include a UE 115-cand a base station 105-c which may be examples of the correspondingdevices described with reference to FIGS. 1-3. The process flow 400 mayinclude aspects for configuring a random access response message (msgB)at the base station 105-c and monitoring channel resources at the UE115-c according to a configured response window. Alternative examples ofthe following may be implemented, where some steps may be performed in adifferent order than described or not performed at all. In someimplementations, steps may include additional features not mentionedbelow, or further steps may be added.

At 405, the UE 115-c may transmit a random access message to the basestation 105-c for connection establishment associated with a randomaccess procedure. The random access message (msgA) may be associatedwith a 2-step random access procedure. For example, the msgAtransmission may include a preamble transmission and a PUSCHtransmission carrying a payload that includes the equivalent contents oraspects of a connectivity request (msg3 of a 4-step random accessprocedure). In some implementations, the UE 115-c may transmit thepreamble and the payload on separate waveforms. The base station 105-cmay receive at least a portion of the msgA transmission at 405 and mayattempt to decode the included payload. In some implementations, thebase station 105-c may receive the preamble transmission and thesubsequent PUSCH transmission associated with the msgA transmission at405.

At 410, the base station 105-c may format a random access responsemessage (msgB) according to the received preamble and PUSCHtransmission. Alternatively, in some other implementations, the basestation 105-c may fail to receive or decode at least one of the preambletransmission or the PUSCH transmission associated with the msgAtransmission 405. For example, the base station 105-c may receive thepreamble transmission but may fail to receive the PUSCH transmission dueto signaling delay associated with contention on the channel. At 410,the base station 105-c may format a random access response message(msgB) based on the inability to receive or decode at least a portion ofthe msgA transmission 405. The configured format of the msgBtransmission may be based on one or more supported response types,including a response for a 2-step random access procedure and a responsefor fallback from the 2-step to a 4-step random access procedure.

At 415, the base station 105-c may transmit the formatted msgBtransmission to the UE 115-c. The formatted msgB transmission maycorrespond to a response type for a 2-step random access transmission ora response type for fallback to a 4-step random access transmission. TheUE 115-c may monitor the channel for reception of the msgB transmission415 according to a configured response window. In some implementations,the UE 115-c may initiate the response window following the PUSCHtransmission associated with the msgA transmission 405. In some otherimplementations, the UE 115-c may initiate the response window followingthe preamble transmission associated with the msgA transmission 405. Theresponse window may exclude the time corresponding to subsequent PUSCHtransmission by the UE 115-c. Based on format of the received msgBtransmission 415, the UE 115-c may determine a connectivityestablishment according to the 2-step random access procedure or anindication of fallback to a 4-step random access procedure.

As part of a fallback communication from a 2-step to a 4-step randomaccess procedure, the UE 115-c may identify an uplink grant of the msgBtransmission 415 and perform retransmission via a msg3 transmission 420.The msg3 transmission 420 may include a UE identifier for contentionresolution. If the UE 115-c is in a connected state, for example, the UEidentifier may be a C-RNTI. Otherwise, the UE identifier may be specificto the UE 115-c.

The base station 105-c may receive the msg 3 retransmission 420 and, inresponse, transmit a msg4 transmission 425. The response may include atleast one of a network identifier of the UE 115-c, a timing advanceparameter, or a backoff indication for the UE 115-c. The backoffindication may include a timing backoff indication or a random accessprocedure backoff indication, or both. The timing backoff indication maybe associated with a timing of the random access procedure and therandom access procedure backoff indication may be associated with thefallback from the 2-step random access procedure to an alternativerandom access procedure (such as a 4-step random access procedure).

FIGS. 5A and 5B illustrate examples of transmission schemes 500-a and500-b that support random access procedure fallback. The transmissionschemes 500-a and 500-b may be implemented for connectivityestablishment between a UE and a base station, as described withreference to FIGS. 1-4. The transmission schemes 500-a and 500-b mayinclude support for random access response message (msgB) or fallbackresponse (msg2) transmission by a base station, as well as support atthe UE for fallback and non-fallback channel response monitoring.

As described herein, the transmission schemes 500-a and 500-b maysupport connection establishment between a base station and a UEaccording to both a 2-step random access procedure and fallback from a2-step to a 4-step random access procedure. When supporting both 2-stepand 4-step random access procedures, in some implementations, the basestation and the UE may commence with one random access procedure (suchas a 2-step random access procedure) and may, in some implementations,fallback to another random access procedure (such as 4-step randomaccess procedure). For example, the base station may support fallbackfrom a 2-step random access procedure via a msg2 transmission as used ina 4-step random access procedure. Additionally, the base station maymultiplex the fallback response with one or more additional msg2transmissions associated with legacy 4-step random access procedures(such as for additional potentially supported UEs).

Based on the capability for msg2 transmission as a fallback response,the UE may monitor the channel for both msgB reception associated with a2-step random access procedure and msg2 reception associated with afallback to a 4-step random access procedure. The UE may monitor thechannel according to one or more response windows (such as responsewindows 515, 520, and 525) and the one or more response windows may beconfigured according to timers. In some implementations, such as acontention-based random access procedure, the UE may transmit thepreamble associated with msgA but fail to transmit the PUSCH payload dueto a loss of contention (due to a signal gap between preamble and PUSCHpayload transmission). As a result, the UE may monitor the one or moreresponse windows for reception of a msg2 transmission associated with afallback response. Alternatively, the UE may transmit the contents oraspects of a msgA transmission for 2-step random access. The UE maymonitor the one or more response windows for reception of a msgBtransmission or a msg2 transmission (as a fallback response due todecoding failure at the base station).

In some implementations, the UE may maintain separate timers associatedwith distinct response windows for monitoring msgB reception andmonitoring msg2 reception. For example, in FIG. 5A, the UE may sense thechannel for initiating random access message (msgA) transmission. Basedon an identification of available channel resources, the UE may transmita preamble transmission 505-a carrying information, such as a UEidentifier. The UE may initiate a response window 515 following thepreamble transmission 505-a. The response window 515 may be configuredfor monitoring the channel for reception of a msg2 transmissionassociated with fallback response. Within or subsequent to the responsewindow 515, the UE may transmit a PUSCH transmission 510-a carrying apayload including a connection request. Following the PUSCH transmission510-a, the UE may initiate a response window 520. The response window520 may be configured for monitoring the channel for reception of a msgBtransmission associated with a 2-step random access procedure.

The response windows 515 and 520 may be configured to overlap over atemporal duration, as part of the channel monitoring. For example, theresponse window 515 may terminate following the initialization of theresponse window 520. Additionally, or alternatively, the response window515 and the response window 520 may be configured to span differenttemporal durations (lengths).

In some implementations, such as FIG. 5B, the UE may maintain a singletimer associated with a single response window. The response window maybe configured for monitoring both msgB reception and msg2 reception. Asdescribed herein, the UE may sense the channel for initiating randomaccess message (msgA) transmission. Based on an identification ofavailable channel resources, the UE may transmit a preamble transmission505-b carrying information. The UE may initiate a response window 525following the preamble transmission 505-b. Prior to the PUSCHtransmission 510-b, the UE may monitor the channel for reception of amsg2 transmission associated with a fallback response. The UE maytransmit the PUSCH transmission 510-b carrying a payload, including aconnection request. Following the PUSCH transmission 510-b, the UE maymonitor the channel for reception of a msg2 transmission associated witha fallback response or a msgB transmission associated with the 2-steprandom access procedure.

In some other implementations, the UE may maintain a single timerassociated with a single response window that follows the PUSCHtransmission 510-b. The response window may be configured for monitoringboth msgB reception and msg2 reception. Additionally, or alternatively,the UE may start and stop monitoring the channel for msg2 receptionearlier than msgB reception.

FIG. 6 illustrates an example of a process flow 600 that supports randomaccess procedure fallback. The process flow 600 may include a UE 115-dand a base station 105-d which may be examples of the correspondingdevices described with reference to FIGS. 1-5. The process flow 600 mayinclude aspects for maintaining one or more response windows at the UE115-d, as part of a support for a 2-step random access procedure or afallback from a 2-step to a 4-step random access procedure.

At 605, the UE 115-d may transmit a random access message to the basestation 105-d for connectivity establishment associated with a randomaccess procedure. The random access message (msgA) may be associatedwith a 2-step random access procedure. For example, the msgAtransmission may include a preamble transmission and a PUSCHtransmission carrying a payload that includes the equivalent contents oraspects of a connectivity request (msg3 of 4-step random accessprocedure). In some implementations, the UE 115-d may transmit thepreamble and the payload on separate waveforms.

At 610, the UE 115-d may monitor the one or more response windows forreceiving a response message following the msgA transmission at 605. Theone or more response windows may be maintained according to configuredtimers and based on the contents or aspects of the msgA transmission at605. In some implementations, the UE 115-d may maintain a first responsewindow for monitoring the channel for msgB reception associated with the2-step random access procedure and a second response window for msg2reception associated with a fallback response. The one or more responsewindows may be configured to span distinct temporal durations and mayoverlap. In some other implementations, the UE 115-d may maintain asingle response window for monitoring both msgB reception associatedwith the 2-step random access procedure and msg2 reception associatedwith a fallback response.

In some implementations, the base station 105-d may receive at least aportion of the msgA transmission at 405 and attempt to decode theincluded payload. In some implementations, the base station 105-d mayreceive the preamble transmission and the subsequent PUSCH transmissionassociated with the msgA transmission at 605.

Alternatively, in some other implementations, the base station 105-d mayfail to receive or decode at least one of the preamble transmission orthe PUSCH transmission associated with the msgA transmission 405. Forexample, the base station 105-d may receive the preamble transmissionbut may fail to receive the PUSCH transmission due to a signaling delayassociated with the contention on the channel.

Based on the reception, at 615, the base station 105-d may performeither msg2 transmission or msgB transmission directed to the UE 115-d.The msgB transmission may be associated with a 2-step random accessprocedure and may be based on successfully receiving the preamble andPUSCH transmissions associated with msgA transmission 605. The msg2transmission may be associated with a fallback procedure and include anuplink grant for subsequent PUSCH payload retransmission.

As part of a fallback communication from a 2-step to a 4-step randomaccess procedure, the UE 115-d may identify an uplink grant of the msg2transmission 615 and perform retransmission via a msg3 transmission 620.The msg3 transmission 620 may include a UE identifier for contentionresolution. If the UE 115-d is in a connected state, for example, the UEidentifier may be a C-RNTI. Otherwise, the UE identifier may be specificto the UE 115-d.

The base station 105-d may receive the msg 3 retransmission 620 and, inresponse, transmit a msg4 transmission 625. The response may include atleast one of a network identifier of the UE 115-d, a timing advanceparameter, or a backoff indication for the UE 115-d. The backoffindication may include a timing backoff indication or a random accessprocedure backoff indication, or both. The timing backoff indication maybe associated with a timing of the random access procedure and therandom access procedure backoff indication may be associated with thefallback from the 2-step random access procedure to an alternativerandom access procedure (such as a 4-step random access procedure).

When supporting both 2-step and 4-step random access procedures, in someimplementations, a base station and a UE may commence with one randomaccess procedure (such as a 2-step random access procedure) and may, insome implementations, fallback to another random access procedure (suchas 4-step random access procedure). For example, the base station maysupport fallback from a 2-step random access procedure via msg2transmission as used in a 4-step random access procedure.

For a 4-step random access procedure, the random access response message(msg2) may have a same or a different configuration (format) compared tothe preceding random access message (msg1). The msg2 transmission maycarry information for the UE via a MAC PDU. The MAC PDU may include anindex of a detected preamble sequence and for which the response isvalid, a timing advance parameter determined based in part on thepreamble sequence, a scheduling grant (such as an uplink grant)indicating time and frequency resources for the UE to use fortransmission, and an assigned network identifier (such as a temporaryC-RNTI) for further communication. In addition, the MAC PDU may includea single reserved bit that is set to 0.

The uplink grant included in the MAC PDU may span multiple bits andinclude one or more indications for subsequent random access message(msg3) transmission. The uplink grant may include one or more RAR grantfields, including a frequency hopping flag, a msg3 PUSCH frequencyresource allocation, a msg3 PUSCH time resource allocation, a modulationand coding scheme (MCS), a transmit power control (TPC) command for msg3PUSCH, and a channel state information (CSI) request, as shown in Table1, reproduced below:

TABLE 1 Number RAR grant field of bits Frequency hopping flag 1 Msg3PUSCH frequency resource 14 allocation Msg3 PUSCH time resource 4allocation MCS 4 TPC command for Msg3 PUSCH 3 CSI request 1

For 4-step random access procedure, the msg3 transmission is the firstPUSCH transmission of the random access procedure. As a result, astatically configured redundancy version (for example, redundancyversion 0) may be used for the PUSCH transmission.

Alternatively, for fallback from a 2-step to a 4-step random accessprocedure, the UE may have already transmitted a PUSCH payload as part arandom access message (msgA). Based on the transmission, the uplinkgrant included in the msg2 transmission (such as for fallback response)may indicate new data indicator or a redundancy version to benefit fromthe incremental redundancy in PUSCH retransmission.

FIG. 7 illustrates an example of a process flow 700 that supports randomaccess procedure fallback. The process flow 700 may include a UE 115-eand a base station 105-e which may be examples of the correspondingdevices described with reference to FIGS. 1-6. The process flow 700 mayinclude aspects for configuring a set of redundancy versions, andindicating a redundancy version for msg3 transmission as part of afallback from a 2-step to a 4-step random access procedure.

In some implementations, the base station 105-e may configure a set ofredundancy versions for a new data transmission or retransmissionassociated with a fallback response. The configured set of new dataindications or redundancy versions may be indicated via a systeminformation indication a master information block (MIB) or systeminformation block (SIB) of an acquisition process, or a remainingminimum system information (RMSI) indication. In some implementations,the set of redundancy versions may span the full set of allowedredundancy versions. In some other implementations, the set maycorrespond to a subset of the allowed redundancy versions. For example,the system information may indicate the redundancy versions (0,3)associated with more systematic bits and fewer parity bits relative tothe alternative (such as (1,2)) redundancy versions. The increasednumber of systematic bits may aid in functionality for self-decoding.

At 705, the UE 115-e may transmit a random access message to the basestation 105-e for connectivity establishment associated with a randomaccess procedure. The random access message (msgA) may be associatedwith a 2-step random access procedure. For example, the msgAtransmission may include a preamble transmission and a PUSCHtransmission carrying a payload that includes the equivalent contents oraspects of a connectivity request (msg3 of a 4-step random accessprocedure). In some implementations, the preamble and the payload may betransmitted on separate waveforms.

The base station 105-e may receive at least a portion of the msgAtransmission 705, and attempt to decode the included payload. In someimplementations, the base station 105-e may fail to receive or decode atleast one of the preamble transmission or the PUSCH transmissionassociated with the msgA transmission 705. For example, the base station105-e may receive the preamble transmission but may fail to receive thePUSCH transmission due to signaling delay associated with contention onthe channel. In some other implementations, the base station 105-e mayreceive the msgA transmission 705 and decode the preamble transmissionbut fail to decode the included PUSCH transmission due to signalattenuation or interference. Based on the failure to receive or thefailure to decode, the base station 105-e may configure a msg2transmission associated with a fallback from a 2-step to a 4-step randomaccess procedure.

For example, at 710, the base station 105-e may determine a redundancyversion indication for performing PUSCH payload retransmission. The basestation 105-e may include the indication in a single reserved bit of theMAC PDU for the msg2 transmission. Due to the formatting of the MAC PDUincluding to a single bit, the base station 105-e may indicate aselected redundancy version (0 or 3) of the subset of configuredredundancy versions. That is, the base station 105-d may use thereserved bit of the fallback response to indicate a redundancy versionfor retransmission. By using the supported MAC PDU format for a 4-steprandom access procedure, the base station 105-e may multiplex thefallback response with one or more additional msg2 transmissionsassociated with legacy 4-step random access procedures.

In some other implementations, at 710, the base station 105-e mayinclude a set of redundancy versions for performing PUSCH payloadretransmission. The base station 105-e may include the set of allowedredundancy versions in the uplink grant configured for the MAC PDU ofthe msg2 transmission. By including the set of allowed redundancyversions, the base station 105-e may change the design of the msg2transmission for random access response. Additionally, or alternatively,the base station 105-e and may configure one or more new data indicatorvalues as part of the formatted msg2 transmission. For example, the basestation 105-e may provide a new MAC PDU structure for the msg2transmission to support indication for the set of allowed redundancyversions along with a new data indicator.

At 715, the base station 105-e may perform msg2 transmission as afallback response associated with the fallback from a 2-step to a 4-steprandom access procedure. The msg2 transmission may include at least anindex of a preamble sequence detected at the base station 105-e for themsgA transmission 705, a timing advance parameter determined based inpart on the preamble sequence detected, and scheduling grant (an uplinkgrant) indicating time and frequency resources for the UE 115-e to usefor PUSCH retransmission via a msg3 transmission. As described herein,the reserved bit included in a MAC PDU associated with the msg2transmission 715 may include an indication of a redundancy version forperforming msg3 transmission. Additionally, or alternatively, the uplinkgrant of the msg2 transmission 715 may signal the set of allowedredundancy versions.

As part of a fallback communication from a 2-step to a 4-step randomaccess procedure, the UE 115-e may identify the uplink grant of the msg2transmission 715 and perform retransmission via a msg3 transmission 720.The msg3 transmission 720 may be formatted according to an indicatedredundancy version included in the msg2 transmission 715. By using theindicated redundancy version, the msg3 transmission 720 may promoteincremental redundancy and enhance capability for potential decoding atthe base station 105-e. The msg3 transmission 720 also may include a UEidentifier for contention resolution. If the UE 115-e is in a connectedstate, for example, the UE identifier may be a C-RNTI. Otherwise, the UEidentifier may be specific to the UE 115-e.

Additionally, or alternatively, the msg3 transmission 720 may include anew data indicator. The new data indicator may include a bit indicationas part of a format for the msg3 transmission 720. In someimplementations, the new data indicator may include a notification ofthe indicated redundancy version associated with retransmission and aspart of the fallback from a 2-step to a 4-step random access procedure.In some other implementations, the new data indicator may include anotification of a new data transmission associated with the msg3transmission 720 between the UE 115-e and the base station 105-e.

The base station 105-e may receive the msg 3 retransmission 720 and, inresponse, transmit a msg4 transmission 725. The response may include atleast one of a network identifier of the UE 115-e, a timing advanceparameter, or a backoff indication for the UE 115-e. The backoffindication may include a timing backoff indication or a random accessprocedure backoff indication, or both. The timing backoff indication maybe associated with a timing of the random access procedure and therandom access procedure backoff indication may be associated with thefallback from the 2-step random access procedure to an alternativerandom access procedure (such as 4-step random access procedure).

FIG. 8 illustrates an example of a process flow 800 that supports randomaccess procedure fallback. The process flow 800 may include a UE 115-fand a base station 105-f which may be examples of the correspondingdevices described with reference to FIGS. 1-7. The process flow 800 mayinclude aspects for configuring a set of redundancy versions, andindicating a redundancy version for msg3 transmission as part of afallback from a 2-step to a 4-step random access procedure.

In some implementations, at 805, the base station 105-f may configure aredundancy version for a new data transmission or retransmissionassociated with a fallback response. The configured redundancy versionmay be indicated via a system information indication a MIB or SIB of anacquisition process, or an RMSI indication. In some otherimplementations, a redundancy version may be statically configured atthe base station 105-f and the UE 115-f based on a standardconfiguration or other specified context.

At 810, the UE 115-f may transmit a random access message to the basestation 105-f for connectivity establishment associated with a randomaccess procedure. The random access message (msgA) may be associatedwith a 2-step random access procedure. For example, the msgAtransmission may include a preamble transmission and a PUSCHtransmission carrying a payload that includes the equivalent contents oraspects of a connectivity request (msg3 of 4-step random accessprocedure). In some implementations, the preamble and the payload may betransmitted on separate waveforms.

The base station 105-f may receive at least a portion of the msgAtransmission at 805 and attempt to decode the included payload. In someimplementations, the base station 105-f may fail to receive or decode atleast one of the preamble transmission or the PUSCH transmissionassociated with the msgA transmission at 805. For example, the basestation 105-f may receive the preamble transmission but may fail toreceive the PUSCH transmission due to signaling delay associated withcontention on the channel. In some other implementations, the basestation 105-f may receive the msgA transmission at 805 and decode thepreamble transmission but fail to decode the included PUSCH transmissiondue to signal attenuation or interference.

At 815, the base station 105-f may perform msg2 transmission as afallback response associated with the fallback from a 2-step to a 4-steprandom access procedure. The msg2 transmission may include at least anindex of a preamble sequence detected at the base station 105-f for themsgA transmission 805, a timing advance parameter determined based inpart on the preamble sequence detected, and scheduling grant (an uplinkgrant) indicating time and frequency resources for the UE 115-f to usefor PUSCH retransmission via a msg3 transmission.

As part of a fallback communication from a 2-step to a 4-step randomaccess procedure, the UE 115-f may identify the uplink grant of the msg2transmission at 815 and perform retransmission via a msg3 transmissionat 820. The msg3 transmission at 820 may be formatted according to theredundancy version indicated or configured (according to a standardconfiguration) for fallback response. The msg3 transmission at 820 alsomay include a UE identifier for contention resolution. If the UE 115-fis in a connected state, for example, the UE identifier may be a C-RNTI.Otherwise, the UE identifier may be specific to the UE 115-f.

Additionally, or alternatively, the msg3 transmission at 820 may includea new data indicator. The new data indicator may include a bitindication as part of a format for the msg3 transmission at 820. In someimplementations, the new data indicator may include a notification ofthe indicated redundancy version associated with retransmission and aspart of the fallback from a 2-step to a 4-step random access procedure.In some other implementations, the new data indicator may include anotification of a new data transmission associated with the msg3transmission at 820. The new data transmission may include a newpreamble transmission for random access procedure between the UE 115-fand the base station 105-f.

The base station 105-f may receive the msg3 retransmission at 820 and,in response, may transmit a msg4 transmission at 825. The response mayinclude at least one of a network identifier of the UE 115-f, a timingadvance parameter, or a backoff indication for the UE 115-f. The backoffindication may include a timing backoff indication or a random accessprocedure backoff indication, or both. The timing backoff indication maybe associated with a timing of the random access procedure and therandom access procedure backoff indication may be associated with thefallback from the 2-step random access procedure to an alternativerandom access procedure (such as 4-step random access procedure).

FIG. 9 illustrates a block diagram 900 of an example device 905 thatsupports random access procedure fallback. The device 905 may be anexample of aspects of a UE 115 as described herein. The device 905 mayinclude a receiver 910, a communications manager 915, and a transmitter920. The device 905 also may include a processor. Each of thesecomponents may be in communication with one another (such as via one ormore buses).

The receiver 910 may receive information such as packets, user data, orcontrol information associated with various information channels (suchas control channels, data channels, and information related to randomaccess procedure fallback, etc.). Information may be passed on to othercomponents of the device 905. The receiver 910 may be an example ofaspects of the transceiver 1220 described with reference to FIG. 12. Thereceiver 910 may utilize a single antenna or a set of antennas.

In some implementations, the UE communications manager 915 may transmit,to a base station, a first random access message of a first randomaccess procedure, the first random access message including a randomaccess preamble and a connection request, and monitor a response windowof a channel to receive a second random access message in response tothe first random access message, the response window based on aconfigured timer. In some implementations, the UE communications manager915 may identify a format of the second random access message based onthe receiving, where the format of the second random access messageindicates one of: the first random access procedure or a second randomaccess procedure, and establish a connection with the base station basedon the first random access message, the second random access message,and the indicated one of the first random access procedure or the secondrandom access procedure. The UE communications manager 915 may be anexample of aspects of the UE communications manager 1210 describedherein.

In some implementations, the UE communications manager 915 may transmit,to a base station, a first random access message of a first randomaccess procedure, the first random access message including a randomaccess preamble and a connection request, and monitor one or moreresponse windows of a channel to receive a second random access messagein response to the first random access message. In some implementations,the UE communications manager 915 may select one of the first randomaccess procedure or a second random access procedure, where theselection is based on a response window of the one or more responsewindows over which the second random access message is received, andestablish a connection with the base station based on the first randomaccess message, the second random access message, and the selected oneof the first random access procedure or the second random accessprocedure. The UE communications manager 915 may be an example ofaspects of the UE communications manager 1210 described herein.

In some implementations, the UE communications manager 915 may transmit,to a base station, a first random access message of a first randomaccess procedure, the first random access message including a randomaccess preamble and a first redundancy version of a connection request,and transmit a third random access message in response to the indicatedswitch from the first random access procedure to the second randomaccess procedure, the third random access message including at least oneof a new data indicator or a second redundancy version of the connectionrequest. In some implementations, the UE communications manager 915 mayreceive a fourth random access message from the base station in responseto the third random access message, the fourth random access messageincluding a connection setup message in response to the connectionrequest, and receive, in response to the first random access message, asecond random access message indicating a switch from the first randomaccess procedure to a second random access procedure. The UEcommunications manager 915 may be an example of aspects of the UEcommunications manager 1210 described herein.

The UE communications manager 915, or its sub-components, may beimplemented in hardware, code (such as software or firmware) executed bya processor, or any combination thereof. If implemented in code executedby a processor, the functions of the UE communications manager 915, orits sub-components may be executed by a general-purpose processor, adigital signal processor (DSP), an application-specific integratedcircuit (ASIC), a field-programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described in the present disclosure.

The UE communications manager 915, or its sub-components, may bephysically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations by one or more physical components. In some implementations,the UE communications manager 915, or its sub-components, may be aseparate and distinct component in accordance with various aspects ofthe present disclosure. In some implementations, the UE communicationsmanager 915, or its sub-components, may be combined with one or moreother hardware components, including but not limited to an input/output(I/O) component, a transceiver, a network server, another computingdevice, one or more other components described in the presentdisclosure, or a combination thereof in accordance with various aspectsof the present disclosure.

The transmitter 920 may transmit signals generated by other componentsof the device 905. In some implementations, the transmitter 920 may becollocated with a receiver 910 in a transceiver module. For example, thetransmitter 920 may be an example of aspects of the transceiver 1220described with reference to FIG. 12. The transmitter 920 may utilize asingle antenna or a set of antennas.

In some examples, the UE communications manager 915 may be implementedas an integrated circuit or chipset for a mobile device modem, and thereceiver 910 and transmitter 920 may be implemented as analog components(such as amplifiers, filters, antennas) coupled with the mobile devicemodem to enable wireless transmission and reception over one or morebands.

The UE communications manager 915 as described herein may be implementedto realize one or more potential advantages. One implementation mayallow the device 905 reduce delay in establishing communication with abase station and avoid prolonged connection procedures, which may resultin fewer transmissions and monitoring occasions.

Based on techniques for efficiently establishing communication with thebase station, the UE communications manager 915 may turn off one or moreprocessing units of device 905 for transmitting and receiving messageswith the base station, reducing the number of computations the UEcommunications manager 915 may perform and therefore increasing powersavings and increasing the battery life of the device 905.

FIG. 10 illustrates a block diagram 1000 of an example device 1005 thatsupports random access procedure fallback. The device 1005 may be anexample of aspects of a device 905, or a UE 115 as described herein. Thedevice 1005 may include a receiver 1010, a UE communications manager1015, and a transmitter 1050. The device 1005 also may include aprocessor. Each of these components may be in communication with oneanother (such as via one or more buses).

The receiver 1010 may receive information such as packets, user data, orcontrol information associated with various information channels (suchas control channels, data channels, and information related to randomaccess procedure fallback, etc.). Information may be passed on to othercomponents of the device 1005. The receiver 1010 may be an example ofaspects of the transceiver 1220 described with reference to FIG. 12. Thereceiver 1010 may utilize a single antenna or a set of antennas.

The UE communications manager 1015 may be an example of aspects of theUE communications manager 915 as described herein. The UE communicationsmanager 1015 may include a random access component 1020, a monitoringcomponent 1025, a format component 1030, a connection component 1035, aselecting component 1040, and a fallback component 1045. The UEcommunications manager 1015 may be an example of aspects of the UEcommunications manager 1210 described herein.

The random access component 1020 may transmit, to a base station, afirst random access message of a first random access procedure, thefirst random access message including a random access preamble and aconnection request.

The random access component 1020 may transmit a third random accessmessage in response to an indicated switch from the first random accessprocedure to the second random access procedure, the third random accessmessage including at least one of a new data indicator or a secondredundancy version of the connection request, and receive a fourthrandom access message from the base station in response to the thirdrandom access message, the fourth random access message including aconnection setup message in response to the connection request.

In some implementations, the monitoring component 1025 may monitor aresponse window of a channel to receive a second random access messagein response to the first random access message, the response windowbased on a configured timer.

In some other implementations, the monitoring component 1025 may monitorone or more response windows of a channel to receive a second randomaccess message in response to the first random access message.

The format component 1030 may identify a format of the second randomaccess message based on the receiving, where the format of the secondrandom access message indicates one of: the first random accessprocedure or a second random access procedure.

The connection component 1035 may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure.

The selecting component 1040 may select one of the first random accessprocedure or a second random access procedure, where the selection isbased on a response window of the one or more response windows overwhich the second random access message is received.

The fallback component 1045 may receive, in response to the first randomaccess message, a second random access message indicating a switch fromthe first random access procedure to a second random access procedure.

The transmitter 1050 may transmit signals generated by other componentsof the device 1005. In some implementations, the transmitter 1050 may becollocated with a receiver 1010 in a transceiver module. For example,the transmitter 1050 may be an example of aspects of the transceiver1220 described with reference to FIG. 12. The transmitter 1050 mayutilize a single antenna or a set of antennas.

FIG. 11 illustrates a block diagram 1100 of an example UE communicationsmanager 1105 that supports random access procedure fallback. The UEcommunications manager 1105 may be an example of aspects of a UEcommunications manager 915, a UE communications manager 1015, or a UEcommunications manager 1210 described herein. The UE communicationsmanager 1105 may include a random access component 1110, a monitoringcomponent 1115, a format component 1120, a connection component 1125, afallback component 1130, a channel sensing component 1135, a selectingcomponent 1140, a timing component 1145, a redundancy version component1150, and a system information component 1155. Each of these modules maycommunicate, directly or indirectly, with one another (such as via oneor more buses).

The random access component 1110 may transmit, to a base station, afirst random access message of a first random access procedure, thefirst random access message including a random access preamble and aconnection request.

In some implementations, the random access component 1110 may transmit,to a base station, a first random access message of a first randomaccess procedure, the first random access message including a randomaccess preamble and a connection request.

In some implementations, the random access component 1110 may transmit athird random access message in response to the indicated switch from thefirst random access procedure to the second random access procedure, thethird random access message including at least one of a new dataindicator or a second redundancy version of the connection request.

In some implementations, the random access component 1110 may receive afourth random access message from the base station in response to thethird random access message, the fourth random access message includinga connection setup message in response to the connection request.

The monitoring component 1115 may monitor a response window of a channelto receive a second random access message in response to the firstrandom access message, the response window based on a configured timer.

In some implementations, the monitoring component 1115 may monitor oneor more response windows of a channel to receive a second random accessmessage in response to the first random access message.

In some implementations, the monitoring component 1115 may initiate theresponse window of the channel following the connection request. In someimplementations, the monitoring component 1115 may initiate the responsewindow of the channel following the random access preamble and prior tothe connection request. In some implementations, the monitoringcomponent 1115 may initiate, following the random access preamble, afirst response window of the one or more response windows.

In some implementations, the monitoring component 1115 may monitor,following the connection request, the response window for receiving thesecond random access message as part of the first random accessprocedure or the second random access procedure. In someimplementations, the monitoring component 1115 may monitor the responsewindow for receiving the second random access message as part of thefirst random access procedure or the second random access procedure.

In some implementations the first response window and the secondresponse window span different temporal durations. In someimplementations, the first response window and the second responsewindow overlap during a temporal duration.

The format component 1120 may identify a format of the second randomaccess message based on the receiving, where the format of the secondrandom access message indicates one of: the first random accessprocedure or a second random access procedure.

In some implementations, determining the second random access messageincludes at least a random access response and a connection setupmessage for the first random access procedure. In some implementations,determining the second random access message includes at least apreamble index and an uplink grant indicating a switch from the firstrandom access procedure to the second random access procedure.

The connection component 1125 may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure. In someimplementations, the connection component 1125 may establish aconnection with the base station based on the response to the connectionrequest.

The fallback component 1130 may receive, in response to the first randomaccess message, a second random access message indicating a switch fromthe first random access procedure to a second random access procedure.In some implementations, the fallback component 1130 may determine thatestablishing the connection is further based on the third random accessmessage and the fourth random access message.

In some implementations, the first random access procedure is a two-steprandom access procedure and the second random access procedure is afour-step random access procedure. In some implementations, the secondrandom access message includes a medium access control protocol dataunit including at least an uplink grant, a timing advance command, anetwork identifier, and a reserved bit.

The channel sensing component 1135 may sense the channel prior to atleast one of the random access preamble or the connection request. Insome implementations, the channel sensing component 1135 may determinetransmitting is based on sensing the channel, the transmitting spanningone or more physical uplink shared channel transmit occasions.

The selecting component 1140 may select one of the first random accessprocedure or a second random access procedure, where the selection isbased on a response window of the one or more response windows overwhich the second random access message is received.

The timing component 1145 may determine the first response window isbased on a first configured timer and the second response window isbased on a second configured timer.

The redundancy version component 1150 may receive an indication of thesecond redundancy version in the second random access message.

In some implementations, the redundancy version component 1150 mayselect the second redundancy version of the connection request for thethird random access message based on the indication. In someimplementations, the redundancy version component 1150 may identify thesecond redundancy version of the connection request based on a standardconfiguration.

The system information component 1155 may identify a subset of a set ofsupported redundancy version identification values is based on areceived remaining minimum system information transmission. In someimplementations, the system information component 1155 may receive abroadcast of system information prior to transmitting the first randomaccess message, the system information identifying a set of supportedredundancy versions for the first random access message or the thirdrandom access message.

FIG. 12 illustrates a diagram of an example system 1200 including adevice 1205 that supports random access procedure fallback. The device1205 may be an example of or include the components of device 905,device 1005, or a UE 115 as described herein. The device 1205 mayinclude components for bi-directional voice and data communicationsincluding components for transmitting and receiving communications,including a UE communications manager 1210, an I/O controller 1215, atransceiver 1220, an antenna 1225, memory 1230, and a processor 1240.These components may be in electronic communication via one or morebuses (such as bus 1245).

In some implementations, the UE communications manager 1210 maytransmit, to a base station, a first random access message of a firstrandom access procedure, the first random access message including arandom access preamble and a connection request, and monitor a responsewindow of a channel to receive a second random access message inresponse to the first random access message, the response window basedon a configured timer. In some implementations, the UE communicationsmanager 1210 may identify a format of the second random access messagebased on the receiving, where the format of the second random accessmessage indicates one of: the first random access procedure or a secondrandom access procedure, and establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure.

In some implementations, the UE communications manager 1210 maytransmit, to a base station, a first random access message of a firstrandom access procedure, the first random access message including arandom access preamble and a connection request, and monitor one or moreresponse windows of a channel to receive a second random access messagein response to the first random access message. In some implementations,the UE communications manager 1210 may select one of the first randomaccess procedure or a second random access procedure, where theselection is based on a response window of the one or more responsewindows over which the second random access message is received, andestablish a connection with the base station based on the first randomaccess message, the second random access message, and the selected oneof the first random access procedure or the second random accessprocedure.

In some implementations, the UE communications manager 1210 maytransmit, to a base station, a first random access message of a firstrandom access procedure, the first random access message including arandom access preamble and a first redundancy version of a connectionrequest, and transmit a third random access message in response to theindicated switch from the first random access procedure to the secondrandom access procedure, the third random access message including atleast one of a new data indicator or a second redundancy version of theconnection request. In some implementations, the UE communicationsmanager 1210 may receive a fourth random access message from the basestation in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request, and receive, in response to the first randomaccess message, a second random access message indicating a switch fromthe first random access procedure to a second random access procedure.

The I/O controller 1215 may manage input and output signals for thedevice 1205. The I/O controller 1215 also may manage peripherals notintegrated into the device 1205. In some examples, the I/O controller1215 may represent a physical connection or port to an externalperipheral. In some examples, the I/O controller 1215 may utilize anoperating system such as iOS®, ANDROID®, MS-DOS®, MS-WINDOWS®, OS/2®,UNIX®, LINUX®, or another known operating system. In some other cases,the I/O controller 1215 may represent or interact with a modem, akeyboard, a mouse, a touchscreen, or a similar device. In some examples,the I/O controller 1215 may be implemented as part of a processor. Insome examples, a user may interact with the device 1205 via the I/Ocontroller 1215 or via hardware components controlled by the I/Ocontroller 1215.

The transceiver 1220 may communicate bi-directionally, via one or moreantennas, wired, or wireless links as described above. For example, thetransceiver 1220 may represent a wireless transceiver and maycommunicate bi-directionally with another wireless transceiver. Thetransceiver 1220 also may include a modem to modulate the packets andprovide the modulated packets to the antennas for transmission, and todemodulate packets received from the antennas.

In some examples, the wireless device may include a single antenna 1225.However, in some examples the device may have more than one antenna1225, which may be capable of concurrently transmitting or receivingmultiple wireless transmissions.

The memory 1230 may include random-access memory (RAM) and read-onlymemory (ROM). The memory 1230 may store computer-readable,computer-executable code 1235 including instructions that, whenexecuted, cause the processor to perform various functions describedherein. In some examples, the memory 1230 may contain, among otherthings, a basic I/O system (BIOS) which may control basic hardware orsoftware operation such as the interaction with peripheral components ordevices.

The processor 1240 may include an intelligent hardware device, (such asa general-purpose processor, a DSP, a central processing unit (CPU), amicrocontroller, an ASIC, an FPGA, a programmable logic device, adiscrete gate or transistor logic component, a discrete hardwarecomponent, or any combination thereof). In some examples, the processor1240 may be configured to operate a memory array using a memorycontroller. In some other cases, a memory controller may be integratedinto the processor 1240. The processor 1240 may be configured to executecomputer-readable instructions stored in a memory (such as the memory1230) to cause the device 1205 to perform various functions (such asfunctions or tasks supporting random access procedure fallback).

The code 1235 may include instructions to implement aspects of thepresent disclosure, including instructions to support wirelesscommunications. The code 1235 may be stored in a non-transitorycomputer-readable medium such as system memory or other type of memory.In some examples, the code 1235 may not be directly executable by theprocessor 1240 but may cause a computer (such as when compiled andexecuted) to perform functions described herein.

FIG. 13 illustrates a block diagram 1300 of an example device 1305 thatsupports random access procedure fallback. The device 1305 may be anexample of aspects of a base station 105 as described herein. The device1305 may include a receiver 1310, a base station communications manager1315, and a transmitter 1320. The device 1305 also may include aprocessor. Each of these components may be in communication with oneanother (such as via one or more buses).

The receiver 1310 may receive information such as packets, user data, orcontrol information associated with various information channels (suchas control channels, data channels, and information related to randomaccess procedure fallback, etc.). Information may be passed on to othercomponents of the device 1305. The receiver 1310 may be an example ofaspects of the transceiver 1620 described with reference to FIG. 16. Thereceiver 1310 may utilize a single antenna or a set of antennas.

In some implementations, the base station communications manager 1315may monitor a channel for receiving, from a UE, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request, anddetermine a format of a second random access message based on receivingthe first random access message, where the format of the second randomaccess message indicates one of: the first random access procedure or asecond random access procedure, transmit, to the UE, the second randomaccess message in response to the first random access message. In someimplementations, the base station communications manager 1315 mayestablish a connection with the base station based on the first randomaccess message, the second random access message, and the indicated oneof the first random access procedure or the second random accessprocedure.

In some implementations, the base station communications manager 1315may monitor a channel for receiving, from a UE, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request, anddetermine a payload of the first random access message based onreceiving the first random access message, transmit, based on thedetermining, a second random access message in response to the firstrandom access message, where the second random access message isassociated with one of: the first random access procedure or a secondrandom access procedure. In some implementations, the base stationcommunications manager 1315 may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the associated one of the first random accessprocedure or the second random access procedure.

In some implementations, the base station communications manager 1315may monitor a channel for receiving, from a UE, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a first redundancyversion of a connection request, and identify an absence of theconnection request or an inability to decode a payload of the firstrandom access message based on the monitoring. In some implementations,the base station communications manager 1315 may transmit, in responseto the first random access message, a second random access messageindicating a switch from the first random access procedure to a secondrandom access procedure, receive a third random access message inresponse to the indicated switch from the first random access procedureto a second random access procedure, the third random access messageincluding at least one of a new data indicator or a second redundancyversion of the connection request, and transmit a fourth random accessmessage in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request. The base station communications manager 1315may be an example of aspects of the base station communications manager1610 described herein.

The base station communications manager 1315, or its sub-components, maybe implemented in hardware, code (such as software or firmware) executedby a processor, or any combination thereof. If implemented in codeexecuted by a processor, the functions of the base stationcommunications manager 1315, or its sub-components may be executed by ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed in the present disclosure.

The base station communications manager 1315, or its sub-components, maybe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations by one or more physical components. In some implementations,the base station communications manager 1315, or its sub-components, maybe a separate and distinct component in accordance with various aspectsof the present disclosure. In some implementations, the base stationcommunications manager 1315, or its sub-components, may be combined withone or more other hardware components, including but not limited to anI/O component, a transceiver, a network server, another computingdevice, one or more other components described in the presentdisclosure, or a combination thereof in accordance with various aspectsof the present disclosure.

The transmitter 1320 may transmit signals generated by other componentsof the device 1305. In some implementations, the transmitter 1320 may becollocated with a receiver 1310 in a transceiver module. For example,the transmitter 1320 may be an example of aspects of the transceiver1620 described with reference to FIG. 16. The transmitter 1320 mayutilize a single antenna or a set of antennas.

FIG. 14 illustrates a block diagram 1400 of an example device 1405 thatsupports random access procedure fallback. The device 1405 may be anexample of aspects of a device 1305, or a base station 105 as describedherein. The device 1405 may include a receiver 1410, a base stationcommunications manager 1415, and a transmitter 1445. The device 1405also may include a processor. Each of these components may be incommunication with one another (such as via one or more buses).

The receiver 1410 may receive information such as packets, user data, orcontrol information associated with various information channels (suchas control channels, data channels, and information related to randomaccess procedure fallback, etc.). Information may be passed on to othercomponents of the device 1405. The receiver 1410 may be an example ofaspects of the transceiver 1620 described with reference to FIG. 16. Thereceiver 1410 may utilize a single antenna or a set of antennas.

The base station communications manager 1415 may be an example ofaspects of the base station communications manager 1315 as describedherein. The base station communications manager 1415 may include amonitoring component 1420, a format component 1425, a random accesscomponent 1430, a connection component 1435, and a fallback component1440. The base station communications manager 1415 may be an example ofaspects of the base station communications manager 1610 describedherein.

In some implementations, the monitoring component 1420 may monitor achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a connection request.

In some other implementations, the monitoring component 1420 may monitora channel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a first redundancy version of a connectionrequest and identify an absence of the connection request or aninability to decode a payload of the first random access message basedon the monitoring.

The format component 1425 may determine a format of a second randomaccess message based on receiving the first random access message, wherethe format of the second random access message indicates one of: thefirst random access procedure or a second random access procedure.

The random access component 1430 may transmit, to the UE, the secondrandom access message in response to the first random access message. Insome implementations, the second random access message may be associatedwith one of: the first random access procedure or a second random accessprocedure.

The random access component 1430 may receive a third random accessmessage in response to the indicated switch from the first random accessprocedure to a second random access procedure, the third random accessmessage including at least one of a new data indicator or a secondredundancy version of the connection request and transmit a fourthrandom access message in response to the third random access message,the fourth random access message including a connection setup message inresponse to the connection request.

The connection component 1435 may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure.

The fallback component 1440 may transmit, in response to the firstrandom access message, a second random access message indicating aswitch from the first random access procedure to a second random accessprocedure.

The transmitter 1445 may transmit signals generated by other componentsof the device 1405. In some implementations, the transmitter 1445 may becollocated with a receiver 1410 in a transceiver module. For example,the transmitter 1445 may be an example of aspects of the transceiver1620 described with reference to FIG. 16. The transmitter 1445 mayutilize a single antenna or a set of antennas.

FIG. 15 illustrates a block diagram 1500 of an example base stationcommunications manager 1505 that supports random access procedurefallback. The base station communications manager 1505 may be an exampleof aspects of a base station communications manager 1315, a base stationcommunications manager 1415, or a base station communications manager1610 described herein. The base station communications manager 1505 mayinclude a monitoring component 1510, a format component 1515, a randomaccess component 1520, a connection component 1525, a configuringcomponent 1530, a fallback component 1535, a multiplexing component1540, a redundancy version component 1545, and a system informationcomponent 1550. Each of these modules may communicate, directly orindirectly, with one another (such as via one or more buses).

The monitoring component 1510 may monitor a channel for receiving, froma UE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request.

In some implementations, the monitoring component 1510 may monitor achannel for receiving, from a UE, a first random access message of afirst random access procedure, the first random access message includinga random access preamble and a first redundancy version of a connectionrequest. In some implementations, the monitoring component 1510 maydetermine a payload of the first random access message based onreceiving the first random access message.

In some implementations, the monitoring component 1510 may identify therandom access preamble and the connection request based on themonitoring. In some implementations, the monitoring component 1510 mayidentify an absence of the connection request or an inability to decodea payload of the first random access message based on the monitoring.

The format component 1515 may determine a format of a second randomaccess message based on receiving the first random access message, wherethe format of the second random access message indicates one of: thefirst random access procedure or a second random access procedure.

The random access component 1520 may transmit, to the UE, the secondrandom access message in response to the first random access message. Insome implementations, the second random access message may be associatedwith one of: the first random access procedure or a second random accessprocedure.

In some implementations, the random access component 1520 may receive athird random access message in response to the indicated switch from thefirst random access procedure to a second random access procedure, thethird random access message including at least one of a new dataindicator or a second redundancy version of the connection request. Insome implementations, the random access component 1520 may transmit afourth random access message in response to the third random accessmessage, the fourth random access message including a connection setupmessage in response to the connection request.

In some implementations, the random access component 1520 may receive,from the UE, a third random access message based on the switch from thefirst random access procedure to the second random access procedure, thethird random access message including a retransmission of the connectionrequest. In some implementations, the random access component 1520 mayindicate, in the second random access message, the second redundancyversion of the connection request for the third random access message.

The connection component 1525 may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure. In someimplementations, the connection component 1525 may establish aconnection with the base station based on the response to the connectionrequest.

The fallback component 1535 may transmit, in response to the firstrandom access message, a second random access message indicating aswitch from the first random access procedure to a second random accessprocedure.

In some implementations, the fallback component 1535 may determine thatestablishing the connection is further based on the third random accessmessage and the fourth random access message. In some implementations,the first random access procedure is a two-step random access procedureand the second random access procedure is a four-step random accessprocedure. In some implementations, the second random access messageincludes a medium access control protocol data unit including at leastan uplink grant, a timing advance command, a network identifier, and areserved bit.

The configuring component 1530 may configure the second random accessmessage to include a random access response and a connection setupmessage for the first random access procedure. In some implementations,the configuring component 1530 may configure the second random accessmessage to include a preamble index and an uplink grant indicating aswitch from the first random access procedure to the second randomaccess procedure.

The multiplexing component 1540 may multiplex the second random accessmessage with one or more additional random access messages for a randomaccess response.

The redundancy version component 1545 may configure a subset of a set ofsupported redundancy versions. In some implementations, the redundancyversion component 1545 may configure a set of supported redundancyversions.

The system information component 1550 may transmit, to the UE, aremaining system information transmission including the subset of theset of supported redundancy versions. In some implementations, thesystem information component 1550 may transmit a broadcast of systeminformation prior to transmitting the first random access message, thesystem information identifying the set of supported redundancy versionsfor the first random access message or the third random access message.

FIG. 16 illustrates a diagram of an example system 1600 including adevice 1605 that supports random access procedure fallback. The device1605 may be an example of or include the components of device 1305,device 1405, or a base station 105 as described herein. The device 1605may include components for bi-directional voice and data communicationsincluding components for transmitting and receiving communications,including a base station communications manager 1610, a network basestation communications manager 1615, a transceiver 1620, an antenna1625, memory 1630, a processor 1640, and an inter-station base stationcommunications manager 1645. These components may be in electroniccommunication via one or more buses (such as bus 1650).

In some implementations, the base station communications manager 1610may monitor a channel for receiving, from a UE, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request,determine a format of a second random access message based on receivingthe first random access message, where the format of the second randomaccess message indicates one of: the first random access procedure or asecond random access procedure, transmit, to the UE, the second randomaccess message in response to the first random access message, andestablish a connection with the base station based on the first randomaccess message, the second random access message, and the indicated oneof the first random access procedure or the second random accessprocedure.

In some implementations, the base station communications manager 1610also may monitor a channel for receiving, from a UE, a first randomaccess message of a first random access procedure, the first randomaccess message including a random access preamble and a connectionrequest, determine a payload of the first random access message based onreceiving the first random access message, transmit, based on thedetermining, a second random access message in response to the firstrandom access message, where the second random access message isassociated with one of: the first random access procedure or a secondrandom access procedure, and establish a connection with the basestation based on the first random access message, the second randomaccess message, and the associated one of the first random accessprocedure or the second random access procedure.

In some implementations, the base station communications manager 1610also may monitor a channel for receiving, from a UE, a first randomaccess message of a first random access procedure, the first randomaccess message including a random access preamble and a first redundancyversion of a connection request, identify an absence of the connectionrequest or an inability to decode a payload of the first random accessmessage based on the monitoring, transmit, in response to the firstrandom access message, a second random access message indicating aswitch from the first random access procedure to a second random accessprocedure, receive a third random access message in response to theindicated switch from the first random access procedure to a secondrandom access procedure, the third random access message including atleast one of a new data indicator or a second redundancy version of theconnection request, and transmit a fourth random access message inresponse to the third random access message, the fourth random accessmessage including a connection setup message in response to theconnection request.

The network base station communications manager 1615 may managecommunications with the core network (such as via one or more wiredbackhaul links). For example, the network base station communicationsmanager 1615 may manage the transfer of data communications for clientdevices, such as one or more UEs 115.

The transceiver 1620 may communicate bi-directionally, via one or moreantennas, wired, or wireless links as described above. For example, thetransceiver 1620 may represent a wireless transceiver and maycommunicate bi-directionally with another wireless transceiver. Thetransceiver 1620 also may include a modem to modulate the packets andprovide the modulated packets to the antennas for transmission, and todemodulate packets received from the antennas.

In some examples, the wireless device may include a single antenna 1625.However, in some examples the device may have more than one antenna1625, which may be capable of concurrently transmitting or receivingmultiple wireless transmissions.

The memory 1630 may include RAM, ROM, or a combination thereof. Thememory 1630 may store computer-readable code 1635 including instructionsthat, when executed by a processor (such as the processor 1640) causethe device to perform various functions described herein. In someexamples, the memory 1630 may contain, among other things, a BIOS whichmay control basic hardware or software operation such as the interactionwith peripheral components or devices.

The processor 1640 may include an intelligent hardware device, (such asa general-purpose processor, a DSP, a CPU, a microcontroller, an ASIC,an FPGA, a programmable logic device, a discrete gate or transistorlogic component, a discrete hardware component, or any combinationthereof). In some examples, the processor 1640 may be configured tooperate a memory array using a memory controller. In some examples, amemory controller may be integrated into processor 1640. The processor1640 may be configured to execute computer-readable instructions storedin a memory (such as the memory 1630) to cause the device 1605 toperform various functions (such as functions or tasks supporting randomaccess procedure fallback).

The inter-station base station communications manager 1645 may managecommunications with other base station 105, and may include a controlleror scheduler for controlling communications with UEs 115 in cooperationwith other base stations 105. For example, the inter-station basestation communications manager 1645 may coordinate scheduling fortransmissions to UEs 115 for various interference mitigation techniquessuch as beamforming or joint transmission. In some implementations, theinter-station base station communications manager 1645 may provide an X2interface within an LTE/LTE-A wireless communication network technologyto provide communication between base stations 105.

The code 1635 may include instructions to implement aspects of thepresent disclosure, including instructions to support wirelesscommunications. The code 1635 may be stored in a non-transitorycomputer-readable medium such as system memory or other type of memory.In some examples, the code 1635 may not be directly executable by theprocessor 1640 but may cause a computer (such as when compiled andexecuted) to perform functions described herein.

FIG. 17 illustrates a flowchart illustrating an example method 1700 thatsupports random access procedure fallback. The operations of method 1700may be implemented by a UE 115 or its components as described herein.For example, the operations of method 1700 may be performed by acommunications manager as described with reference to FIGS. 9-12. Insome implementations, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally, or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 1705, the UE may transmit, to a base station, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request. Theoperations of 1705 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 1705 maybe performed by a random access component as described with reference toFIGS. 9-12.

At 1710, the UE may monitor a response window of a channel to receive asecond random access message in response to the first random accessmessage, the response window based on a configured timer. The operationsof 1710 may be performed according to the methods described herein. Insome implementations, aspects of the operations of 1710 may be performedby a monitoring component as described with reference to FIGS. 9-12.

At 1715, the UE may identify a format of the second random accessmessage based on the receiving, where the format of the second randomaccess message indicates one of: the first random access procedure or asecond random access procedure. The operations of 1715 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 1715 may be performed by a format componentas described with reference to FIGS. 9-12.

At 1720, the UE may establish a connection with the base station basedon the first random access message, the second random access message,and the indicated one of the first random access procedure or the secondrandom access procedure. The operations of 1720 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 1720 may be performed by a connectioncomponent as described with reference to FIGS. 9-12.

FIG. 18 illustrates a flowchart illustrating an example method 1800 thatsupports random access procedure fallback. The operations of method 1800may be implemented by a UE 115 or its components as described herein.For example, the operations of method 1800 may be performed by acommunications manager as described with reference to FIGS. 9-12. Insome implementations, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally, or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 1805, the UE may transmit, to a base station, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request. Theoperations of 1805 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 1805 maybe performed by a random access component as described with reference toFIGS. 9-12.

At 1810, the UE may monitor a response window of a channel to receive asecond random access message in response to the first random accessmessage, the response window based on a configured timer. The operationsof 1810 may be performed according to the methods described herein. Insome implementations, aspects of the operations of 1810 may be performedby a monitoring component as described with reference to FIGS. 9-12.

At 1815, the UE may identify a format of the second random accessmessage based on the receiving, where the format of the second randomaccess message indicates one of: the first random access procedure or asecond random access procedure. The operations of 1815 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 1815 may be performed by a format componentas described with reference to FIGS. 9-12.

At 1820, the UE may transmit a third random access message based on theswitch from the first random access procedure to the second randomaccess procedure, the third random access message including aretransmission of the connection request. The operations of 1820 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 1820 may be performed by arandom access component as described with reference to FIGS. 9-12.

At 1825, the UE may receive a fourth random access message from the basestation in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request. The operations of 1825 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 1825 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

At 1830, the UE may establish a connection with the base station basedon the first random access message, the second random access message,and the indicated one of the first random access procedure or the secondrandom access procedure. The operations of 1830 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 1830 may be performed by a connectioncomponent as described with reference to FIGS. 9-12.

FIG. 19 illustrates a flowchart illustrating an example method 1900 thatsupports random access procedure fallback. The operations of method 1900may be implemented by a UE 115 or its components as described herein.For example, the operations of method 1900 may be performed by acommunications manager as described with reference to FIGS. 9-12. Insome implementations, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally, or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 1905, the UE may transmit, to a base station, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request. Theoperations of 1905 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 1905 maybe performed by a random access component as described with reference toFIGS. 9-12.

At 1910, the UE may monitor one or more response windows of a channel toreceive a second random access message in response to the first randomaccess message. The operations of 1910 may be performed according to themethods described herein. In some implementations, aspects of theoperations of 1910 may be performed by a monitoring component asdescribed with reference to FIGS. 9-12.

At 1915, the UE may select one of the first random access procedure or asecond random access procedure, where the selection is based on aresponse window of the one or more response windows over which thesecond random access message is received. The operations of 1915 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 1915 may be performed by aselecting component as described with reference to FIGS. 9-12.

At 1920, the UE may establish a connection with the base station basedon the first random access message, the second random access message,and the selected one of the first random access procedure or the secondrandom access procedure. The operations of 1920 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 1920 may be performed by a connectioncomponent as described with reference to FIGS. 9-12.

FIG. 20 illustrates a flowchart illustrating an example method 2000 thatsupports random access procedure fallback. The operations of method 2000may be implemented by a UE 115 or its components as described herein.For example, the operations of method 2000 may be performed by acommunications manager as described with reference to FIGS. 9-12. Insome implementations, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally, or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 2005, the UE may transmit, to a base station, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a connection request. Theoperations of 2005 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2005 maybe performed by a random access component as described with reference toFIGS. 9-12.

At 2010, the UE may monitor one or more response windows of a channel toreceive a second random access message in response to the first randomaccess message. The operations of 2010 may be performed according to themethods described herein. In some implementations, aspects of theoperations of 2010 may be performed by a monitoring component asdescribed with reference to FIGS. 9-12.

At 2015, the UE may select one of the first random access procedure or asecond random access procedure, where the selection is based on aresponse window of the one or more response windows over which thesecond random access message is received. The operations of 2015 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 2015 may be performed by aselecting component as described with reference to FIGS. 9-12.

At 2020, the UE may transmit a third random access message to the basestation based at least in part selecting the second random accessprocedure, the third random access message including a retransmission ofthe connection request. The operations of 2020 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2020 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

At 2025, the UE may receive a fourth random access message from the basestation in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request. The operations of 2025 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2025 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

At 2030, the UE may establish a connection with the base station basedon the first random access message, the second random access message,and the selected one of the first random access procedure or the secondrandom access procedure. The operations of 2030 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2030 may be performed by a connectioncomponent as described with reference to FIGS. 9-12.

FIG. 21 illustrates a flowchart illustrating an example method 2100 thatsupports random access procedure fallback. The operations of method 2100may be implemented by a UE 115 or its components as described herein.For example, the operations of method 2100 may be performed by acommunications manager as described with reference to FIGS. 9-12. Insome implementations, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally, or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 2105, the UE may transmit, to a base station, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a first redundancyversion of a connection request. The operations of 2105 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2105 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

At 2110, the UE may receive, in response to the first random accessmessage, a second random access message indicating a switch from thefirst random access procedure to a second random access procedure. Theoperations of 2110 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2110 maybe performed by a fallback component as described with reference toFIGS. 9-12.

At 2115, the UE may transmit a third random access message in responseto the indicated switch from the first random access procedure to thesecond random access procedure, the third random access messageincluding at least one of a new data indicator or a second redundancyversion of the connection request. The operations of 2115 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 2115 may be performed by arandom access component as described with reference to FIGS. 9-12.

At 2120, the UE may receive a fourth random access message from the basestation in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request. The operations of 2120 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2120 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

FIG. 22 illustrates a flowchart illustrating an example method 2200 thatsupports random access procedure fallback. The operations of method 2200may be implemented by a UE 115 or its components as described herein.For example, the operations of method 2200 may be performed by acommunications manager as described with reference to FIGS. 9-12. Insome implementations, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally, or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 2205, the UE may transmit, to a base station, a first random accessmessage of a first random access procedure, the first random accessmessage including a random access preamble and a first redundancyversion of a connection request. The operations of 2205 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2205 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

At 2210, the UE may receive, in response to the first random accessmessage, a second random access message indicating a switch from thefirst random access procedure to a second random access procedure. Theoperations of 2210 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2210 maybe performed by a fallback component as described with reference toFIGS. 9-12.

At 2215, the UE may transmit a third random access message in responseto the indicated switch from the first random access procedure to thesecond random access procedure, the third random access messageincluding at least one of a new data indicator or a second redundancyversion of the connection request. The operations of 2215 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 2215 may be performed by arandom access component as described with reference to FIGS. 9-12.

At 2220, the UE may receive a fourth random access message from the basestation in response to the third random access message, the fourthrandom access message including a connection setup message in responseto the connection request. The operations of 2220 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2220 may be performed by a random accesscomponent as described with reference to FIGS. 9-12.

At 2225, the UE may establish a connection with the base station basedon the response to the connection request. The operations of 2225 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 2225 may be performed by aconnection component as described with reference to FIGS. 9-12.

FIG. 23 illustrates a flowchart illustrating an example method 2300 thatsupports random access procedure fallback. The operations of method 2300may be implemented by a base station 105 or its components as describedherein. For example, the operations of method 2300 may be performed by acommunications manager as described with reference to FIGS. 13-16. Insome implementations, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally, or alternatively, a basestation may perform aspects of the functions described below usingspecial-purpose hardware.

At 2305, the base station may monitor a channel for receiving, from aUE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request. The operations of 2305 may be performed according tothe methods described herein. In some implementations, aspects of theoperations of 2305 may be performed by a monitoring component asdescribed with reference to FIGS. 13-16.

At 2310, the base station may determine a format of a second randomaccess message based on receiving the first random access message, wherethe format of the second random access message indicates one of: thefirst random access procedure or a second random access procedure. Theoperations of 2310 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2310 maybe performed by a format component as described with reference to FIGS.13-16.

At 2315, the base station may transmit, to the UE, the second randomaccess message in response to the first random access message. Theoperations of 2315 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2315 maybe performed by a random access component as described with reference toFIGS. 13-16.

At 2320, the base station may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure. The operations of 2320may be performed according to the methods described herein. In someimplementations, aspects of the operations of 2320 may be performed by aconnection component as described with reference to FIGS. 13-16.

FIG. 24 illustrates a flowchart illustrating an example method 2400 thatsupports random access procedure fallback. The operations of method 2400may be implemented by a base station 105 or its components as describedherein. For example, the operations of method 2400 may be performed by acommunications manager as described with reference to FIGS. 13-16. Insome implementations, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally, or alternatively, a basestation may perform aspects of the functions described below usingspecial-purpose hardware.

At 2405, the base station may monitor a channel for receiving, from aUE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request. The operations of 2405 may be performed according tothe methods described herein. In some implementations, aspects of theoperations of 2405 may be performed by a monitoring component asdescribed with reference to FIGS. 13-16.

At 2410, the base station may identify an absence of the connectionrequest or an inability to decode a payload of the first random accessmessage based on the monitoring. The operations of 2410 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2410 may be performed by a monitoringcomponent as described with reference to FIGS. 13-16.

At 2415, the base station may configure the second random access messageto include a preamble index and an uplink grant indicating a switch fromthe first random access procedure to the second random access procedure.The operations of 2415 may be performed according to the methodsdescribed herein. In some implementations, aspects of the operations of2415 may be performed by a configuring component as described withreference to FIGS. 13-16.

At 2420, the base station may determine a format of a second randomaccess message based on receiving the first random access message, wherethe format of the second random access message indicates one of: thefirst random access procedure or a second random access procedure. Theoperations of 2420 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2420 maybe performed by a format component as described with reference to FIGS.13-16.

At 2425, the base station may transmit, to the UE, the second randomaccess message in response to the first random access message. Theoperations of 2425 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2425 maybe performed by a random access component as described with reference toFIGS. 13-16.

At 2430, the base station may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the indicated one of the first random accessprocedure or the second random access procedure. The operations of 2430may be performed according to the methods described herein. In someimplementations, aspects of the operations of 2430 may be performed by aconnection component as described with reference to FIGS. 13-16.

FIG. 25 illustrates a flowchart illustrating an example method 2500 thatsupports random access procedure fallback. The operations of method 2500may be implemented by a base station 105 or its components as describedherein. For example, the operations of method 2500 may be performed by acommunications manager as described with reference to FIGS. 13-16. Insome implementations, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally, or alternatively, a basestation may perform aspects of the functions described below usingspecial-purpose hardware.

At 2505, the base station may monitor a channel for receiving, from aUE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request. The operations of 2505 may be performed according tothe methods described herein. In some implementations, aspects of theoperations of 2505 may be performed by a monitoring component asdescribed with reference to FIGS. 13-16.

At 2510, the base station may determine a payload of the first randomaccess message based on receiving the first random access message. Theoperations of 2510 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2510 maybe performed by a monitoring component as described with reference toFIGS. 13-16.

At 2515, the base station may transmit, based on the determining, asecond random access message in response to the first random accessmessage, where the second random access message is associated with oneof: the first random access procedure or a second random accessprocedure. The operations of 2515 may be performed according to themethods described herein. In some implementations, aspects of theoperations of 2515 may be performed by a random access component asdescribed with reference to FIGS. 13-16.

At 2520, the base station may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the associated one of the first random accessprocedure or the second random access procedure. The operations of 2520may be performed according to the methods described herein. In someimplementations, aspects of the operations of 2520 may be performed by aconnection component as described with reference to FIGS. 13-16.

FIG. 26 illustrates a flowchart illustrating an example method 2600 thatsupports random access procedure fallback. The operations of method 2600may be implemented by a base station 105 or its components as describedherein. For example, the operations of method 2600 may be performed by acommunications manager as described with reference to FIGS. 13-16. Insome implementations, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally, or alternatively, a basestation may perform aspects of the functions described below usingspecial-purpose hardware.

At 2605, the base station may monitor a channel for receiving, from aUE, a first random access message of a first random access procedure,the first random access message including a random access preamble and aconnection request. The operations of 2605 may be performed according tothe methods described herein. In some implementations, aspects of theoperations of 2605 may be performed by a monitoring component asdescribed with reference to FIGS. 13-16.

At 2610, the base station may determine a payload of the first randomaccess message based on receiving the first random access message. Theoperations of 2610 may be performed according to the methods describedherein. In some implementations, aspects of the operations of 2610 maybe performed by a monitoring component as described with reference toFIGS. 13-16.

At 2615, the base station may identify an absence of the connectionrequest or an inability to decode the payload of the first random accessmessage based on the monitoring. The operations of 2615 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2615 may be performed by a monitoringcomponent as described with reference to FIGS. 13-16.

At 2620, the base station may configure the second random access messageto include a preamble index and an uplink grant indicating a switch fromthe first random access procedure to the second random access procedure.The operations of 2620 may be performed according to the methodsdescribed herein. In some implementations, aspects of the operations of2620 may be performed by a configuring component as described withreference to FIGS. 13-16.

At 2625, the base station may transmit, based on the determining, asecond random access message in response to the first random accessmessage, where the second random access message is associated with oneof: the first random access procedure or a second random accessprocedure. The operations of 2625 may be performed according to themethods described herein. In some implementations, aspects of theoperations of 2625 may be performed by a random access component asdescribed with reference to FIGS. 13-16.

At 2630, the base station may establish a connection with the basestation based on the first random access message, the second randomaccess message, and the associated one of the first random accessprocedure or the second random access procedure. The operations of 2630may be performed according to the methods described herein. In someimplementations, aspects of the operations of 2630 may be performed by aconnection component as described with reference to FIGS. 13-16.

FIG. 27 illustrates a flowchart illustrating an example method 2700 thatsupports random access procedure fallback. The operations of method 2700may be implemented by a base station 105 or its components as describedherein. For example, the operations of method 2700 may be performed by acommunications manager as described with reference to FIGS. 13-16. Insome implementations, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally, or alternatively, a basestation may perform aspects of the functions described below usingspecial-purpose hardware.

At 2705, the base station may monitor a channel for receiving, from aUE, a first random access message of a first random access procedure,the first random access message including a random access preamble and afirst redundancy version of a connection request. The operations of 2705may be performed according to the methods described herein. In someimplementations, aspects of the operations of 2705 may be performed by amonitoring component as described with reference to FIGS. 13-16.

At 2710, the base station may identify an absence of the connectionrequest or an inability to decode a payload of the first random accessmessage based on the monitoring. The operations of 2710 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2710 may be performed by a monitoringcomponent as described with reference to FIGS. 13-16.

At 2715, the base station may transmit, in response to the first randomaccess message, a second random access message indicating a switch fromthe first random access procedure to a second random access procedure.The operations of 2715 may be performed according to the methodsdescribed herein. In some implementations, aspects of the operations of2715 may be performed by a fallback component as described withreference to FIGS. 13-16.

At 2720, the base station may receive a third random access message inresponse to the indicated switch from the first random access procedureto a second random access procedure, the third random access messageincluding at least one of a new data indicator or a second redundancyversion of the connection request. The operations of 2720 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 2720 may be performed by arandom access component as described with reference to FIGS. 13-16.

At 2725, the base station may transmit a fourth random access message inresponse to the third random access message, the fourth random accessmessage including a connection setup message in response to theconnection request. The operations of 2725 may be performed according tothe methods described herein. In some implementations, aspects of theoperations of 2725 may be performed by a random access component asdescribed with reference to FIGS. 13-16.

FIG. 28 illustrates a flowchart illustrating an example method 2800 thatsupports random access procedure fallback. The operations of method 2800may be implemented by a base station 105 or its components as describedherein. For example, the operations of method 2800 may be performed by acommunications manager as described with reference to FIGS. 13-16. Insome implementations, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally, or alternatively, a basestation may perform aspects of the functions described below usingspecial-purpose hardware.

At 2805, the base station may monitor a channel for receiving, from aUE, a first random access message of a first random access procedure,the first random access message including a random access preamble and afirst redundancy version of a connection request. The operations of 2805may be performed according to the methods described herein. In someimplementations, aspects of the operations of 2805 may be performed by amonitoring component as described with reference to FIGS. 13-16.

At 2810, the base station may identify an absence of the connectionrequest or an inability to decode a payload of the first random accessmessage based on the monitoring. The operations of 2810 may be performedaccording to the methods described herein. In some implementations,aspects of the operations of 2810 may be performed by a monitoringcomponent as described with reference to FIGS. 13-16.

At 2815, the base station may transmit, in response to the first randomaccess message, a second random access message indicating a switch fromthe first random access procedure to a second random access procedure.The operations of 2815 may be performed according to the methodsdescribed herein. In some implementations, aspects of the operations of2815 may be performed by a fallback component as described withreference to FIGS. 13-16.

At 2820, the base station may receive a third random access message inresponse to the indicated switch from the first random access procedureto a second random access procedure, the third random access messageincluding at least one of a new data indicator or a second redundancyversion of the connection request. The operations of 2820 may beperformed according to the methods described herein. In someimplementations, aspects of the operations of 2820 may be performed by arandom access component as described with reference to FIGS. 13-16.

At 2825, the base station may transmit a fourth random access message inresponse to the third random access message, the fourth random accessmessage including a connection setup message in response to theconnection request. The operations of 2825 may be performed according tothe methods described herein. In some implementations, aspects of theoperations of 2825 may be performed by a random access component asdescribed with reference to FIGS. 13-16.

At 2830, the base station may establish a connection with the basestation based on the response to the connection request. The operationsof 2830 may be performed according to the methods described herein. Insome implementations, aspects of the operations of 2830 may be performedby a connection component as described with reference to FIGS. 13-16.

It should be noted that the methods described herein describe possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Further, aspects from two or more of the methods may be combined.

An OFDMA system may implement a radio technology such as Ultra MobileBroadband (UMB), E-UTRA, IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, Flash-OFDM, etc. UTRA and E-UTRA are part of Universal MobileTelecommunications System (UMTS). LTE, LTE-A, and LTE-A Pro are releasesof UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A, LTE-A Pro, NR,and GSM are described in documents from the organization named “3rdGeneration Partnership Project” (3GPP). CDMA2000 and UMB are describedin documents from an organization named “3rd Generation PartnershipProject 2” (3GPP2). The techniques described herein may be used for thesystems and radio technologies mentioned herein as well as other systemsand radio technologies. While aspects of an LTE, LTE-A, LTE-A Pro, or NRsystem may be described for purposes of example, and LTE, LTE-A, LTE-APro, or NR terminology may be used in much of the description, thetechniques described herein are applicable beyond LTE, LTE-A, LTE-A Pro,or NR applications.

A macro cell generally covers a relatively large geographic area (suchas several kilometers in radius) and may allow unrestricted access byUEs with service subscriptions with the network provider. A small cellmay be associated with a lower-powered base station, as compared with amacro cell, and a small cell may operate in the same or different (suchas licensed, unlicensed, etc.) frequency bands as macro cells. Smallcells may include pico cells, femto cells, and micro cells according tovarious implementations. A pico cell, for example, may cover a smallgeographic area and may allow unrestricted access by UEs with servicesubscriptions with the network provider. A femto cell also may cover asmall geographic area (such as a home) and may provide restricted accessby UEs having an association with the femto cell (such as UEs in aclosed subscriber group (CSG), UEs for users in the home, and the like).An eNB for a macro cell may be referred to as a macro eNB. An eNB for asmall cell may be referred to as a small cell eNB, a pico eNB, a femtoeNB, or a home eNB. An eNB may support one or multiple (such as two,three, four, and the like) cells, and also may support communicationsusing one or multiple component carriers.

The wireless communications systems described herein may supportsynchronous or asynchronous operation. For synchronous operation, thebase stations may have similar frame timing, and transmissions fromdifferent base stations may be approximately aligned in time. Forasynchronous operation, the base stations may have different frametiming, and transmissions from different base stations may not bealigned in time. The techniques described herein may be used for eithersynchronous or asynchronous operations.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: a, b, or c” is intended to cover: a, b, c,a-b, a-c, b-c, and a-b-c.

The various illustrative logics, logical blocks, modules, circuits andalgorithm processes described in connection with the implementationsdisclosed herein may be implemented as electronic hardware, computersoftware, or combinations of both. The interchangeability of hardwareand software has been described generally, in terms of functionality,and illustrated in the various illustrative components, blocks, modules,circuits and processes described above. Whether such functionality isimplemented in hardware or software depends upon the particularapplication and design constraints imposed on the overall system.

The hardware and data processing apparatus used to implement the variousillustrative logics, logical blocks, modules and circuits described inconnection with the aspects disclosed herein may be implemented orperformed with a general purpose single- or multi-chip processor, a DSP,an ASIC, an FPGA or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, or, any conventionalprocessor, controller, microcontroller, or state machine. A processoralso may be implemented as a combination of computing devices, such as acombination of a DSP and a microprocessor, multiple of microprocessors,one or more microprocessors in conjunction with a DSP core, or any othersuch configuration. In some implementations, particular processes andmethods may be performed by circuitry that is specific to a givenfunction.

In one or more aspects, the functions described may be implemented inhardware, digital electronic circuitry, computer software, firmware,including the structures disclosed in this specification and theirstructural equivalents thereof, or in any combination thereof.Implementations of the subject matter described in this specificationalso can be implemented as one or more computer programs, one or moremodules of computer program instructions, encoded on a computer storagemedia for execution by, or to control the operation of, data processingapparatus.

If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. The processes of a method or algorithmdisclosed herein may be implemented in a processor-executable softwaremodule which may reside on a computer-readable medium. Computer-readablemedia includes both computer storage media and communication mediaincluding any medium that can be enabled to transfer a computer programfrom one place to another. A storage media may be any available mediathat may be accessed by a computer. By way of example, and notlimitation, such computer-readable media may include RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that may be used to storedesired program code in the form of instructions or data structures andthat may be accessed by a computer. Also, any connection can be properlytermed a computer-readable medium. Disk and disc, as used herein,includes compact disc (CD), laser disc, optical disc, digital versatiledisc (DVD), floppy disk, and Blu-ray disc where disks usually reproducedata magnetically, while discs reproduce data optically with lasers.Combinations of the above should also be included within the scope ofcomputer-readable media. Additionally, the operations of a method oralgorithm may reside as one or any combination or set of codes andinstructions on a machine readable medium and computer-readable medium,which may be incorporated into a computer program product.

Various modifications to the implementations described in thisdisclosure may be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. Thus, the claims are not intended to be limited to theimplementations shown herein, but are to be accorded the widest scopeconsistent with this disclosure, the principles and the novel featuresdisclosed herein.

Additionally, a person having ordinary skill in the art will readilyappreciate, the terms “upper” and “lower” are sometimes used for ease ofdescribing the figures, and indicate relative positions corresponding tothe orientation of the figure on a properly oriented page, and may notreflect the proper orientation of any device as implemented.

Some features that are described in this specification in the context ofseparate implementations also can be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation also can be implemented inmultiple implementations separately or in any suitable subcombination.Moreover, although features may be described above as acting in somecombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Further, the drawings may schematically depict one more exampleprocesses in the form of a flow diagram. However, other operations thatare not depicted can be incorporated in the example processes that areschematically illustrated. For example, one or more additionaloperations can be performed before, after, simultaneously, or betweenany of the illustrated operations. In some circumstances, multitaskingand parallel processing may be advantageous. Moreover, the separation ofvarious system components in the implementations described above shouldnot be understood as requiring such separation in all implementations,and it should be understood that the described program components andsystems can generally be integrated together in a single softwareproduct or packaged into multiple software products. Additionally, otherimplementations are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results.

1. A method for wireless communication at a user equipment (UE),comprising: transmitting, as part of a first random access procedure, afirst random access message including a random access preamble and aconnection request, wherein a redundancy version of the connectionrequest is statically configured and is for transmission of theconnection request and for retransmission of the connection request; andmonitoring a response window of a channel to receive a second randomaccess message in response to the first random access message, theresponse window associated with a configured timer, wherein the secondrandom access message indicates one of: successful first random accessmessage reception or a fallback procedure.
 2. The method of claim 1,wherein the second random access message comprises at least a randomaccess response and a connection setup message, the random accessresponse and the connection setup message indicating the successfulfirst random access message reception.
 3. The method of claim 1, whereinthe second random access message comprises at least a preamble index andan uplink grant, the preamble index and the uplink grant indicating thefallback procedure.
 4. The method of claim 3, further comprising:transmitting a third random access message in accordance with the secondrandom access message indicating the fallback procedure, the thirdrandom access message comprising the retransmission of the connectionrequest, wherein the redundancy version of the third random accessmessage is statically configured for the retransmission of theconnection request; and receiving a fourth random access message fromthe base station in response to the third random access message, thefourth random access message comprising a connection setup message inresponse to the connection request.
 5. The method of claim 4, whereintransmitting the third random access message is associated with theredundancy version of the connection request.
 6. The method of claim 1,wherein monitoring the response window of the channel further comprises:monitoring the response window of the channel following the transmissionof the connection request.
 7. The method of claim 1, further comprising:establishing a connection with a base station based, at least in part,on the first random access message and the second random access message.8. The method of claim 1, wherein the redundancy version comprises aredundancy version number
 0. 9. The method of claim 1, wherein thesuccessful first random access message reception is associated with atwo-step random access procedure and the fallback procedure isassociated with a four-step random access procedure.
 10. The method ofclaim 1, wherein the connection request comprises a radio resourcecontrol (RRC) connection request.
 11. An apparatus for wirelesscommunication at a user equipment (UE), comprising: a processor; memorycoupled with the processor; and instructions stored in the memory andexecutable by the processor to cause the apparatus to: transmit, as partof a first random access procedure, a first random access messageincluding a random access preamble and a connection request, wherein aredundancy version of the connection request is statically configuredand is for transmission of the connection request and for retransmissionof the connection request; and monitor a response window of a channel toreceive a second random access message in response to the first randomaccess message, the response window associated with a configured timer,wherein the second random access message indicates one of: successfulfirst random access message reception or a fallback procedure.
 12. Theapparatus of claim 11, wherein the second random access messagecomprises at least a random access response and a connection setupmessage, the random access response and the connection setup messageindicating the successful first random access message reception.
 13. Theapparatus of claim 11, wherein the second random access messagecomprises at least a preamble index and an uplink grant, the preambleindex and the uplink grant indicating the fallback procedure.
 14. Theapparatus of claim 13, wherein the instructions are further executableby the processor to cause the apparatus to: transmit a third randomaccess message in accordance with the second random access messageindicating the fallback procedure, the third random access messagecomprising the retransmission of the connection request, wherein theredundancy version of the third random access message is staticallyconfigured for the retransmission of the connection request; and receivea fourth random access message in response to the third random accessmessage, the fourth random access message comprising a connection setupmessage in response to the connection request.
 15. The apparatus ofclaim 14, wherein transmitting the third random access message isassociated with the redundancy version of the connection request. 16.The apparatus of claim 11, wherein the instructions to monitor theresponse window of the channel are further executable by the processorto cause the apparatus to: monitor the response window of the channelfollowing the transmission of the connection request.
 17. The apparatusof claim 11, wherein the instructions are further executable by theprocessor to cause the apparatus to: establish a connection with a basestation based, at least in part, on the first random access message andthe second random access message.
 18. The apparatus of claim 11, whereinthe redundancy version comprises a redundancy version number
 0. 19. Theapparatus of claim 11, wherein the successful first random accessmessage reception is associated with a two-step random access procedureand the fallback procedure is associated with a four-step random accessprocedure.
 20. The apparatus of claim 11, wherein the connection requestcomprises a radio resource control (RRC) connection request.
 21. Anapparatus for wireless communication at a user equipment (UE),comprising: means for transmitting, as part of a first random accessprocedure, a first random access message including a random accesspreamble and a connection request, wherein a redundancy version of theconnection request is statically configured and is for transmission ofthe connection request and for retransmission of the connection request;and means for monitoring a response window of a channel to receive asecond random access message in response to the first random accessmessage, the response window associated with a configured timer, whereinthe second random access message indicates one of: successful firstrandom access message reception or a fallback procedure.
 22. Theapparatus of claim 21, wherein the second random access messagecomprises at least a random access response and a connection setupmessage, the random access response and the connection setup messageindicating the successful first random access message reception.
 23. Theapparatus of claim 21, wherein the second random access messagecomprises at least a preamble index and an uplink grant, the preambleindex and the uplink grant indicating the fallback procedure.
 24. Theapparatus of claim 23, further comprising: means for transmitting athird random access message in accordance with the second random accessmessage indicating the fallback procedure, the third random accessmessage comprising the retransmission of the connection request, whereinthe redundancy version of the third random access message is staticallyconfigured for the retransmission of the connection request; and meansfor receiving a fourth random access message in response to the thirdrandom access message, the fourth random access message comprising aconnection setup message in response to the connection request.
 25. Theapparatus of claim 24, wherein transmitting the third random accessmessage is associated with the redundancy version of the connectionrequest.
 26. The apparatus of claim 21, wherein the means for monitoringthe response window of the channel further comprise: means formonitoring the response window of the channel following the transmissionof the connection request.
 27. The apparatus of claim 21, furthercomprising: means for establishing a connection with a base stationbased, at least in part, on the first random access message and thesecond random access message.
 28. The apparatus of claim 21, wherein theredundancy version comprises a redundancy version number
 0. 29. Theapparatus of claim 21, wherein the successful first random accessmessage reception is associated with a two-step random access procedureand the fallback procedure is associated with a four-step random accessprocedure.
 30. A non-transitory computer-readable medium storing codefor wireless communication at a user equipment (UE), the code comprisinginstructions executable by a processor to: transmit, as part of a firstrandom access procedure, a first random access message including arandom access preamble and a connection request, wherein a redundancyversion of the connection request is statically configured and is fortransmission of the connection request and for retransmission of theconnection request; and monitor a response window of a channel toreceive a second random access message in response to the first randomaccess message, the response window associated with a configured timer,wherein the second random access message indicates one of: successfulfirst random access message reception or a fallback procedure.